Originally Posted by notfastenough
So the subs (2 of them) have to be small, cheap, loud with low extension. The magical tall order that everyone would love to have filled.
That's what it sounds like to me. As asarose247 pointed out, Hoffman's Law is still Iron...
The PA460 and RSS390HF both are rated for 500W RMS, but the PA460 will be about 10dB louder in the mid-bass for the same power.
When it comes to the lowest frequencies, displacement sets a limit. One high excursion 15" vs two mid-excursion 18" isn't going to be that different. Isn't the rule of thumb that one UM18 is roughly equivalent to three PA460s? The excursion and prices reflect that. The PA460 is considered a "value" driver, you get a lot of the price. But the UM18 is a "value" driver, too!
Same for the JBL 12" drivers (when on sale). Roughly four equal one UM18.
With these drivers, we're already at the "sweet spot". Bigger cabinets (more VBSS, horns, bandpass) will get you more dB, but every Hz lower you want to extend will always cost more.
I'd recommend selling the RSS390HF and adding another two VBSS since they're simple, you've already built two, and you can run them off your existing amp that already has the settings dialed in.
FWIW I've got "value" subs in my system, a mix of Stereo Integrity HT18s, UM18s, and VBSSs. But I run the UM18s and PA460s nearfield, so they don't have to run as hard. And with eight 18" subs, half of them nearfield (and not counting all the 18s in the main speakers), I'm still wanting to add a big sealed sub in the corner. One way or another, you have to pay to play.