Two subwoofers

Simple question: If you have two subwoofers in the same system properly placed apart and balanced in the room, should they be exactly the same models, amp types and and have matching power outputs? Is it possible if you have two different types that they could be out of sync? For example, if one is a class AB analog amp at one power output and the other a class D digital amp at a higher rated power output, could they be out of sync even slightly?
Yes it is possible but careful volume matching, phasing, and cancellation issues must be addressed. I am currently using five subs(3 different mfr's) in my main HT/music system and believe me, it takes some work and patience to get it "right". I am using a pair of Klipsch RT-10d's in the front corners, a Klipsch RSW-10d next to my recliner, and SVS SB13+ next to my sofa, and also a Revel B15. Different driver sizes(10"-13"-15"), amp powers(500w, 700w, 1000w, 1000w). And yes, my bride is a keeper.

When you match the output level of a sub to the main speakers in a system, you do so at a given SPL (75db in the latest Audyssey set-up regimen IIRC). I think the OP may be asking what happens as that SPL changes. Might two different sub models behave differently as SPL rises.

Theoretically, at least, I believe that the the answer should be "yes". In every sub (and speaker, for that matter) some amplifier output is converted to acoustic output and some is lost to heat - thermal compression. The ratio of the two may differer between different designs.

In practical use, I doubt that it's likely to be meaningful (or even audible). Since most dual sub set-ups aren't stereo, the theoretical mismatch wouldn't affect stereo balance (the subs are operating in mono, anyway). The bigger theoretical issue is how thermal compression impacts the combined output of the subs vs the combined output of the main speakers as the SPL increases. This is as true with identical model subs as it is with differing subs.

It may vary case by case, but I've measured the response of my system (with identical twin Rythmik subs) at 75db and at 95db and the difference is negligible. FWIW.
Thanks all. Wow 5 subs in a single HT system. Whoa and I thought two was more than most. Yes that can be quite tricky. Bob what I meant by out of synch is that perhaps the two different subs were not in exact phase. Perhaps the drivers could be out of the exact timing being of different types of amps (class B and Class D) and one having a larger enclosure. So here's what I ended up doing. I took home a 2nd ASW610 brand new from my dealer and removed the ASW600.

After many hours of listening and trials, I became convinced this was the right move. I purchased the new ASW610 and redeployed my older ASW600 as a sub for my Dell desktop PC in another room. The two ASW610s work extremely well together being very compatible. The LFE and bass extension is far more tight and focused on the sweet spot. I realized that the older ASW610 produces more rattle and actually more vibrations in other rooms. Now those vibrations have greatly reduced which was also my interest in upgrading. So far, mission accomplished !!
I am assuming when you mean out of synch, you are referring to out of phase? I am guessing one sub set at 180 and the other 0. I pondered similar issues a few years ago. My old 15" sealed 1000 watt sub watt (B&W Nautilus ASW 850) has been a workhorse in my system. I wanted to get a replacement but of course they are not made anymore. After researching and doing comparative analysis (Internet sources and manufacturer products), I opted to get another 15" 1000 watt sealed sub, from HSU. It had all of the basic functionality of the B&W in terms of specs and controls. It was easy to blend both of these subs with a Velodyne SMS1 sub eq. Excellent room response. I have them configured directly to the right and left of the main speakers. You will need to level match them first in order to ensure that the volume is the same for a given output.

Good luck,
I use a pair of Velodyne HGS-15s controlled by an SMS-1. I recently bought a pair of SMS-1s with the thought that I might try an SMS-1 for each HGS-15 and use the third SMS-1 in another setup with an HGS-10. I can't quite get my head around the efficacy of a separate SMS-1 for each HGS-15, because it seems that it's the combined output of the subs that one wants to equalize. Nevertheless, the Manual for the SMS-1 covers using them in tandem, so I plan to try it.