After sorting out my room and listening position, I've finally got my 5200s sounding their best. Feeling confident, I have added my SW1600 without the aid of the G61R. (I previously tried using a G61R config which included a music sub and played around with different crossover points, whilst really trying to get around some problems with room nodes which repositioning (largely) solved. I didn't like the sound using the processor to control the bass crossover. Maybe now I've solved some of the fundamental problems it would be a better option...)
My current configuration is MC200 -> MS200 -> Master 5200SL to slave 5200SL to AC12 to SW1600. For some reason when I tried this with the M6s in place of the 5200s I had no volume control on the SW1600 but it works great with the 5200s. SW1600 set to 1+2. Filter switched in.
So the 5200s have been left on Free and the sub filter set to almost to min (40 something Hz - trial and error after listening to several different music types). Gain is set much lower than when I use it (via G61R) for movies. Some may prefer the gain a little higher and calibration may suggest that it should be... But letting my brain calibrate things caters for my own bias!
Fills the low registers nicely and integrates well. Surprisingly easy. Doesn't boom and doesn't sound unnatural.
The real work was running MRC about 20 times one weekend to repeatedly find the best room configuration (including speaker placement) by looking at the room frequency response for each positional setup of the main L R pair. After tweaking positions I find MRC useful but not essential... So running 2 channel with the G61R out of the chain is now my preferred serious music listening mode. It means I have a couple of Speakerlink cables each running to the master and sub and these need swapping over when switching back and forth from running the MS200 as the source or the G61R.
I haven't yet run the front pair and sub as they are, but with the G61R as source with the 5200HC added running in Trifield with the M6s at the sides. It's my next step and I expect it will sound fine. I do like Trifield for quite a lot of my music but I have found that 2 channel sounds slightly more natural. Probably due to my 5200HC being positioned too low for the ideal soundstage, being boxed in under the TV.
I had originally followed the advice in the DSP set up manual and placed the 5200s 0.25m from the wall. They are now approx 0.75m from the wall. For the ultimate sound I have to remove the seating cushions from 2 of my sofas at the sides of the room (they cause a significant dip at 80Hz). Incidentally placing those same cushions behind the listening position against the opposing wall to the speakers further tames the 40Hz bass peak my room introduces.
I may try the sub with my 5000s just for fun.
Incidentally the room is taking a real pounding when playing dance music at 75 and above. Turning up the gain (but not increasing the filter cut off) on the sub is definitely advised for party mode ;-)
So does any of this answer the question of whether or not you should get better DSPs or just add a sub?
Probably not, at least until I run the sub with the 5000s. Although mine are 18 bit and the 5200s are (now) in a league above so I'm not sure my comments would be valid.
But there is no doubt that if you can get it to integrate nicely, a sub will help. I have a REL sub which I used to use for movies but was never happy with. The SW1600 has been a real pleasure and it just integrates so well in Movie mode via the G61R. And now it seems it is handy for music too, although I have to wait until my wife isn't around before I wheel it out to the position where it works best for music! (Trip hazard and eye sore)