The more subs you add in your room the flatter the nodes become, which is the objective to controlling room acoustics. The most benefit comes from adding the second sub which is why most designers suggest buying subs in pairs. Its diminishing returns as you add more subs but each one helps. Adding subs of different models can be done successfully as long as care is taken in getting them in phase which is more easily done if the subs are of the same model. Mixing a 350 and a 500 or 600 can be done with careful placement and paying attention to the polarity switch. While the 350 won't give you the range of a 600 it will combine to smooth nodes over its operating band. The key is proper placement so as not to cancel or accentuate nodes. Currently, I have a Velodyne DD-18 and a pair of EP600s in my HT and its sounding very good across all my HT seats. At other times I've also had an SVS Ultra in the mix. One sub usually sounds really fine only at the sweet spot.

Another example of what can be done is with HSU's MBM which reproduces in the 50-150hz range and is esssentially an add-on bass module to their main deeper subs. IMO a 125/175/350 can be used in much the same way to accent a 500/600, as long as its dialled in and placed properly in the room. Because of boundary reinforcement in the low and ultra bass most room would benefit from higher output in the 30hz and up band anyway. Its one reason why the EP subs come with trim adjustment to boost output above 33hz.

So my take is while its best to add another EP600 in your room, a less expensive alternative which will improve acoustics for someone like you is to add a 125/175/350 elsewhere in the room, maybe beside your seats like HSU recommends with their MBM.

John