I have a remarkably similar system: Def Tech Mythos STS mains, CS8060-HD center, Gem XL surrounds, and Dual SVS PB13-Ultra's. I currently use Audyssey MultEQXT32 in my AVR for all the room correction, but I used to use (and still own) an Antimode 8033-C. I've played around with various bass configurations, including running the STS's as large and LFE+Main, STS as Large but not running LFE+Main (I'm blanking on what to call this, basically not double bass), STS's as small with the crossover set at (at various times), 40, 60, 80, 90, and 100 hz, running the Anti-Mode on all four subs simultaneously via y-splitters (multiple splitters, obviously), and just running the Ultra's off the Antimode with the STS's set as small.
Here are a couple of my thoughts (take them with a grain of salt as room acoustics and preferences are going to be highly individualized):
I was able to achieve the best (smoothest) bass response with all four "subs" running off of a single LFE out and split between the 4 speakers, but this had the least amount of extension. My guess was/is the extension was being effected due to phase cancellations under the STS's tuning point. In my case, per REW (sorry, I no longer have the graphs, so this is off of memory), bass started rolling off rapidly in the low 30hz range. Until that, the Antimode was able to give me a very flat frequency response and it sounded great. But I missed the extension I paid for with my subs.
I was/am able to get the best extension when running the STS's as small and having all the LFE routed to the subs, which were EQ'd by the Antimode. In my room, running the Ultra's in 15hz tuning, I am pretty flat to ~13hz up until about 105-106db, at which point compression starts kicking in on the lowest frequencies, but I am still pretty flat to 16hz up above 110-111 db, and flat to 22hz up to 115-116db. My room provides some gain, but is very leaky, about 4,300 cubic feet, and very awkwardly shaped, all of which impact my results. DB readings were/are via REW ("Room EQ Wizard", a freeware program available from Home Theater Shack) and a Ratshack meter with the appropriate calibration files loaded. When I tried running all 4 "subs" these figures were very different. I had much less extension, and was not able to get anywhere near these max numbers before I heard the STS's crying for mercy.
For me, in my room and to my tastes, the best deep bass and midbass came about when crossing over the STS's (and the 8060HD) to the Ultra's at 100hz. A lot of people are now going to state that the STS's bass sections are useless, they're wrong. I've heard the Mythos One's, and I've tried my Gem XL's as mains, and even crossed over they do not have the same sound stage, impact, or scale as the STS's. My tastes are based off of my room, and my placement, and these resulted in a best crossover point of 100hz. I suggest people try out various frequencies as 80hz is not the best setting for everyone's room and/or equipment.
Setting the bass knobs and getting the transition between the passive and active sections of the Def Tech's took me the longest amount of time during setup and tweaking. Def Tech normally advises people to set the knob at around the 12 o'clock settings and adjust from there. This may work for some, but I ended up using the RTA function of REW to tweak the knob until I got the smoothest transition. Others may not be as OCD as me about this. If looking at REW, which I highly recommend even when using auto eq's such as Audyssey and/or the Antimode, there is a new option to use a USB mic from MiniDSP called the UMIK-1
, which is a calibrated mic and REW now has some neat features to support it. I have not tried the UMIK-1 yet, (need to find the time to play around), but there are threads on it here and at Home Theater Shack. It looks promising, easy, and cheaper than previous/alternate setups.
The owners threads for SVS, Def Tech Mythos, and the Antimode all have good information in them and are great resources for specific issues. Your friend may want to peruse them.
If (S)he has an AVR or pre/pro with room EQ software, run the Antimode first, then the room EQ with the Antimode on and in the system. The Antimode adds a bit of delay to the signal, which shows up as increased distance for the LFE channel, and should be compensated for via the distance setting in the bass management settings (speaker config/distance). Do not worry/and or "adjust" the distance to the sub based off of a tape measure, as this will effect phase and cause problems. Sometimes, with some room eq's, adjusting the distance can be useful for smoothing out the frequency range around the crossover (especially with Audyssey based systems), but this is best determined and adjusted using a program such as REW.
The STS's are very good speakers, but can be more challenging to set up due to the active bass sections. There is no reason though that using them and the SVS would prevent or limit the need for using sub EQ. I'd suggest to your friend the usual pieces of advice: placement and set up are going to be key to getting the best out of his system, room acoustics are a major factor, and room treatments/bass traps/EQ can go a long way towards making a room have less impact on the system.