Thanks for adding to the thread, @eugene81 . That's useful information. I suspect that most of my listening centers around 85db, or at least that's were it sat on the day I checked. I listen in a nearfield position because of the size of my room and so that I may mitigate some of the room's acoustic problems. Of course, much of it is highly dynamic classical or jazz, so I'm getting up to adjust my preamp frequently.
Thanks to advice from @mglik , I purchased a used but low hours/refurbished Audible Illusions M3B. I can't overstate the difference between my Mytek Brooklyn Bridge connected directly to the SIT-3 and the new arrangement. Hard to say the objective sound of the M3B because my only reference point is my previous setup. Perhaps a bit darker sounding? The tubes do impart their magic. I find myself turning the system up a little louder than previously when playing vinyl to reach the sweet spot- and the separation/clarity of instruments. Streaming never requires the gain controls above noonish, but I can get close to the extent of the knobs' travel when playing quieter records. Overall, a great improvement.
Still, a Linear Tube Audio microZOTL is on my list for the future. I went with the M3B because the price was right, and the phono stage is good. When I make the move to a MC cartridge, I will weigh purchasing the gold phono card vs a whole new preamp.
@eugene81 I'm glad you mentioned the Bel Canto Ref 500S, because that unit and the mono block 600M were enticing. I think the one you had actually delivers more current than the monoblocks, and Maggies seem to be current hungry. But, I guess I can scratch those off my list!
Thanks for the recommendation of the XA25. I purchased the SIT-3 first because I was motivated by the limited supply (initially only 250?). As a collector of all things music, I rarely sell back instruments/amps, etc. The SIT-3 is special, and even if I temporarily take it out of the system, it was my first high end audio piece and will probably be with me forever. I will consider the XA25. If I change anything, or add options, it would probably be a high wattage amp. I know conventional wisdom is Bryston with Maggies, but I think I need a more engaging/magical/euphonic amp. So, that hunt continues, but I should reiterate that I'm pretty happy with where I'm at, thanks to advice received here.
I took a power amp from my live bass guitar rig- a QSC CX302- as an experiment. It's 325 watts/channel @ 4 ohms- 10 times the power. The fan was so ridiculously loud that it was laughable. It was nice having more power on hand, but upon switching back to the SIT-3 after a few weeks, it was no contest. I may not have the best tonal memory when switching pieces in and out, so quantifying differences is difficult. Upon its return to my system, the SIT-3 thrilled me from note one, and it pulled me into the music, compelling me to listen for hours longer than I anticipated. One lesson for me is that I will hear a clear audible difference between gear, sometimes showing distaste when it is outside what I'm familiar with. It is through subsequent, in depth listening that I can truly value the swapped gear. The first hours of the M3B were such a shock that I was unable to discern if I liked it!
Thanks for all the input- you guys are the best! I'm realizing much like life, this is a journey rather than an ultimate destination. I'm geeking out on Quad 57s (haven't seen much anecdotal regarding the SIT-3 and Quads) and to a slightly lesser degree, Spatial Audio. Most importantly, having fun and deriving much satisfaction from listening.