3500 cubes is a decent sized space to fill with an IB.
I've got about 2600 cubes, and I built a quad 18 manifold IB, so I'm aware of what you're facing. For starters, you need to consider, like you stated, this is for music 100%. This changes your IB extension requirement somewhat. You don't need the 10hz infra playback that HTs can benefit from. That said, with the HP filter, you can safely utilize much more power for transient peaks, than the same IB for HT duty. That way, you'll have all the peak voltage for demanding material, yet no fear of bottoming that most IB users must always be mindful of.
In no circumstance would I use a plate amp. The Behringer EP2000, EP4000, INukes, all are so inexpensive, and ideally suited. Again, with the HP flter, the exact power level of the amp isn't an issue. I prefer any EQ'ing (HPing) be performed outside the amp, but there are DSP amp options like INuke, CrownXLS, etc.
The SI drives appear nice, but not a great deal of builds yet.
Are you familiar with Fi? Their IB-3 series is ideal for IB, and high performance, high value. I use four of the IB3-18, for around $250 a piece and 30mmXmax, they're well vetted, tough to beat. They're rated at 550w@20hz, so a pair would mate well with an EP2000, a set of four best with an EP4000.
The Dayton 18s would be great as well. Be mindful you'll need some EQ, and measurment and time/phase manipulation gear to align properly in time with mains. This will be critical, and the avenue to great sound. Audyssey or equivalent can do ok, but ideally some outboard time and freq EQing, and a modest measurment rig would be best.
Also, remember, your mains capability will dictate the real capability needed in the sub system. Peak demands are one thing, but unless your system is way out of balance spectrally, bass heavy, you'll only need whatever SPL your mains can produce at the lower end of their operating range.
Ar you considering a manifold, or a front loaded array?
Best of luck
Flat, Deep, Clean, Linear, and Loud
Active 16.8kw, 7.3 system
(3)Seaton Cat12C up front, (4)QSC K8 sides/rears
(2)Seaton SubM-HP, (4)18" IB