After a full day of tweaking and experimenting I can say I am very happy with the Rhumba Extreme!
To recap & follow up on what was done after initial setup, where bass appeared to be restrained...
I am now plugging the Qutest directly into the REC OUT, which bypasses Inputs 1-4, since the Qutest is my only source. It did help with cleaning up a bit of the softness and restoring some snap and detail to the sound.
After initial setup on the glass top of the BDI Corridor I learned that glass tops were not recommended for placement. The BLRE went on a wooden shelf in the cabinet and the bass did focus a bit more and overall sound was cleaner and smoother. HOWEVER, because I have my Orbi satellite and Hue Bridge close to that shelf, I noticed some motorboating (digital cycling noise) in the background. So I stole my wife's cutting board and using IsoAcoustic Oreas I replaced the BLRE on the top of the cabinet. This did result in a massive improvement in the overall sound.
I also eliminated the high level speaker wire connection from the MQ-88uSE to the eSub and went directly from the OUT 2 of the preamp into the sub. This restored a great deal of balance to the bottom end. I also did some further experimenting with the crossover and phase settings of the eSub, moving the crossover from 70 down to 60. Resulted in a much better blend with the La Scala IIs and eliminated some directionality I hadn't noticed before.
All of these tweaks got me to the point where I no longer felt the need to swap the L509X back in to compare - currently I have solid, well-controlled bass with presence and heft, better transients and cleaner background. This combined with the strengths of the BLRE over the 509X I already mentioned (massive soundstage with pinpoint placement accuracy, well-defined space around the notes, more musical unspooling of the music/greater liquidity) have me in a happy place where I am pretty confident I'll be able to let the 509X go to a new home, and explore further gains through tube rolling with the Rhumba Extreme.
Fit and finish of the Rhumba Extreme is top-notch. It's not an exceptionally heavy unit, but it is a solid, attractive, exquisitely machined piece. The toggles are satisfying to flip around and the remote is a heavy piece of jewelry - the best remote I've ever owned from any component. I'm really glad I was able to restore placement on the top of the cabinet with the wood cutting board because this is not a component you want to hide away in a cabinet.
The final piece of the puzzle was the delivery of a new Hugo M Scaler late in the afternoon. Although not related to the Rhumba Extreme, the gains are jaw-dropping when feeding the Qutest dual BNC inputs at full resolution. Suddenly all boundaries between the listener and the music disappear. It's like a portal has opened to the soundstage; it's almost unreal how much of an improvement it makes. It does, on some recordings - particularly with male vocals - add a bit of thinness to the sound I've experienced when upsampling from Roon in the past. But I chalk that up to the limitations of the Qutest more than anything else, and I can definitely see a DAVE upgrade in the next year.
A note about the M Scaler and Rhumba Extreme - I should clarify that I'm not intending to project the gains of the M Scaler to the Rhumba Extreme. When putting the M Scaler in bypass mode, listening to the non-upscaled audio through the system is still remarkably transparent, engaging, and dynamic.
All-in-all this is very close to end-game for me. In fact if it wasn't for the tiniest bit of thinness in male vocals when upscaling to the Qutest, I'd probably not even think about further upgrading to the DAVE, but knowing me, I'll always be nagged that there's "one last step" to achieve. I'm excited about doing some tube rolling with the Rhumba Extreme, but now we're talking the last 1% of gains in this system. At the budget level of most of these components (around $5K each between pre, amp, digital front end, etc) I cannot see experimenting with $3000 power cables or the like. Maybe down the road I end up swapping out the La Scala IIs for Volti Aluras, but for me this is definitely a good place to hit the pause button on the gear sickness and start enjoying music for a while!
