An update for anyone who finds this in a future search - while the 3490 looked gorgeous sitting along side my avr7550, it did not work out for my two subs.
Once setup, level matched / room eq ran - For music and lower volume movies and TV no issues whatsoever. Sound and performance were excellent, though no different than the bash plate amps (not that I was expecting it to). I have read that most plate amp have some sort of eq or boost built in but I heard no difference - room eq probably doing its job. Using the dual sub outs from my avr to the 3490's main amp in worked flawlessly.
The problem was when the volume was very high, on demanding movie soundtracks the receiver would go into protect mode. To intentionally torture test the 3490 I was using the 50cal scene from the last Rambo film at a ridiculous high volume. It would get a few seconds into the heavy bass before protection tripped and shutdown.
The impedance of the drivers on my subs must drop a bit too low for the 3490 to stay happy under extreme demands.
So, I ended up ordering a crown xls1000 . My avr was able to drive the xls just fine, the gain knobs for each channel do have to be 1click below max for the room eq to set the sub channels at +5db in my room (with initial run with gain knobs at 1/2 room eq (ezset/eq 2 on my avr) did not fail but set the sub channels at avr's max of +10db, for comparison original bash plates with gain at 2/3 would result in -1db and the 3490 +1db ) so the xls is definitely expecting higher input voltage, but it works fine and no need for bump box. Was afraid of possible high noise floor I read in some threads/reviews (typically for loudspeaker duty) but subs are silent even with my ear right up to drivers with or without low pass filter in use.
Under the same torture testing the xls1000 performed without a hiccup. Awesome power from this lightweight, but it sure is ugly looking. The red clip lights would just start to flicker on at the most demanding points, but unlike the 3490 it will keep on playing. I left clip limiters engaged to protect drivers. Upgrading to the xls1500 would probably eliminate this, but as it would never occur under any normal listening conditions I will just stick with the 1000.
Music and normal movie and tv sound and performance again seem the same as the original plate amps.
As the xls of course has no auto-on feature I am using a trigger out from my avr to switch power to the xls with relay. Works better than auto-on anyway. If you don't have a trigger output another option is a smart strip.
All in all I'm happy with how it turned out, same sound/performance and less expensive than replacing both sub plates and the crown xls should be more reliable/last longer than the bash plates. I do wish the 3490 would have worked out as it looked awesome on the stand next to my matching HK AVR, and I already had it, but oh well. Would have required relocating several components for permanent install anyway, which would have been a pain. I have made some simple brackets so the xls can stand vertically behind my stand and other components, hidden from view - so it all looks the same from the front. I even wired up the power leds on the subs to the avr trigger with resistors inline so they still come on with the avr/amp.