If you have measuring gear then you could try double bass(which means sub active and mains also full range) and see if FR improved or not compared to setting them to small with crossover.
Typically though, a capable sub will do far better than most "tower" speakers. The placement of the speakers in the room, however, has a big impact on bass response and the overall room modes. So experimentation will typically yield the best results compared to rote dogma.
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I personally would set the center on large... and your towers on small (if you are planning on running your subwoofer all of the time) otherwise, set the towers on large (if the towers can handle the full frequency) also, it is really going to depend on your listening habits. Cheers and Happy Listening!
Your Polk Tsx250c is capable and appears to be able to play a full range signal. Set it to large (listen to it for a bit) then set it to small, and see what you think of the differences... have fun man!
We are all here to give and get advice, and that's all...(LOL)
As mentioned earlier; set all speakers to SMALL (recommended by Audyssey). Then all signals below cross-over frequency from all channels will be sent to sub. Double-bass (or LFE+Main) is not recommended, since this will send low frequencies to both sub and main front speakers. Which of course can be kind of iffy regarding sub + main canceling each other out.
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TSX 250c is down 3db at 58hz (polk specs) so it is Definetely NOT full range.
Set it to small.
"2liter" if you are gonna give advice please get it right.
Just because a speaker is physically "large" doesn't mean it should be set to Large. The decision to send a speaker a full range signal or to apply a crossover should be made based on the low frequency extension of the speaker. If you send a speaker a signal it is not capable of reproducing, that signal is LOST, never to be heard. If, instead, you re-route that signal to a different speaker, (the subwoofer), that IS capable of reproducing it, you'll actually get to hear it.
The Polk TSx250C being discussed in this thread has a -3 dB point of 58 Hz. It's a ported speaker, so it will roll off at 24 dB/octave below 58 Hz. That "spec" is likely measured at 75 dB. If one measured the -3 dB point of that speaker at full Reference Level of 105 dB, that -3 dB point would more likely rise to 80 to 100 Hz, or even higher. Also, the room interaction with the speaker will affect the -3 dB point and the maximum output capability. There is about 1.5 octaves below 58 Hz down to 20 Hz, not to mention any infrasonic content that might be placed in the center channel. If you send content below 58 Hz to that speaker, you'll hear virtually NONE of it. OTOH, if you re-route it to the subwoofer, you'll hear as much of that content as the sub is capable of reproducing, (which presumably is far greater than the output of the speaker.) There is NO DOUBT, it would be advisable to set that speaker to Small, and set a crossover on it. 80 to 100 Hz would likely be optimal. (Actually, at 105 dB those ports will be chuffing away making bad noises at port tune, so it's even less advisable to send these speakers a full range signal.)
The Polk TSx550T mains present some different considerations. They have a -3 dB point of 36 Hz. Even that is not a true "full range" speaker as there is a whole octave from 40 down to 20 Hz, and 2 more octaves below that down to 5 Hz. There is plenty of content recorded in the L/R channels in movies down to those frequencies. If you send these speakers any of that content you will hear virtually NONE of it. Re-routing it to the subwoofer will provide a much better chance of hearing it, (depending on the capability of the sub.) The main speakers do have deeper extension than the CC, and therefore it might be possible to use a lower crossover than the one used for the CC. However, it is still advisable to use a crossover, but one could experiment with 40, 60 or 80 Hz crossovers. (Personally, I would probably opt for an 80 Hz crossover, but I would base that off of in-room response measurements and listening tests.)
There are several other reasons for using crossovers and re-routing the deepest bass to the sub:
Amplifier Headroom: The deepest bass consumes the most power. It takes four times as much power to go 1 octave lower in frequency. It takes 4X as much power to reproduce 40 Hz as it does 80 Hz. That eats up amplifier headroom quickly. You can free up significant amplifier headroom by re-routing the bass to the sub. Subs generally have their own dedicated amps that are more powerful than the individual amp channels in a receiver. Re-routing the bass to the subs allows the entire system to playback louder, cleaner and with less distortion. It also protects the speakers from amplifier clipping.
Placement Issues: Bass drivers are highly influenced by their in-room placement. The peaks and nulls caused by room reflections can result in 20 dB or greater differences in frequency response anomalies. Some of these can be mitigated by changing the placement of the drivers in the room. However, speakers need to go where they need to go for imaging and presenting the optimal soundstage. They leave little flexibility for adjustment to optimize the bass response. The sub, OTOH, can be moved around to find the best possible location for the "transfer function" of the sound to the listening position. Re-routing all the bass to the sub allows that bass to be reproduced optimally in the room.
EQ Issues: EQ can be applied to a single subwoofer output much more effectively than trying to EQ each main channel individually. Systems like Audyssey, (depending on the version of Audyssey in use), can do a better job of EQ'ing a single subwoofer channel, with all the bass sent to that single channel, than it can trying to EQ each speaker's bass individually.
Overall, I can think of no reason why the speakers in the OP's system should be set to Large/Full Range. For the multitude of reasons above, applying crossovers to all the channels will result in a much more optimal system, one that can playback louder and cleaner, and with better sound quality, than a system without crossovers. His system includes a subwoofer. It ought to be the best bass reproduction speaker in his system. He should use it for the reason he bought it... to reproduce the bass in his system.
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You gave bad advice. He doesn't have a center speaker that reaches down to 20hz. My center speaker goes lower than his, and I still set mine to small.
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