PREAMPLIFIER UPGRADE QUESTION

@ronboco  - is there a Boulder preamp under $7500? Didn't think they had products at that price point...

+2 Backert Labs. I have a Rhumba Extreme 1.3 set up with a Pass XA30.8. I really enjoy the combination.

Thanks for the additional info — most helpful.  I was gonna recommend the Backert Rhumba Extreme as well, but the current version is $8500 so a bit above the OP’s budget unless buying used.  Personally, of your choices I’d choose the Rogue over the Bryston because you may find it tough going from tubes to solid state, and the RP9 is certainly no slouch.  I had a Bryston BP6 for a while and it was a great pre but would not consider it to have that “tube magic”.  Not to complicate things further (actually that’s exactly what I’m doing here), but in your price range I’d absolutely have both the Spatial Audio Raven and Aric Audio Motherload XL high on my radar and would choose either over the Rogue, but that’s me.  Just my $0.02 FWIW, and best of luck.

I had RP-1 with ST-100 driving Martin Logan Montis.
Rogue is great value and RP-1 is excellent. Not familiar with the sound of the bryston amp and how it pairs with the ML speakers but I would find a Bryston preamp on a used market and try it. Synergy is key in a system set up. 
Buy used and try. 
 

One other recommendation if I may add…Brimar CV4003 tubes from Upscale Audio took my RP-1 preamp to a new level. Cheap upgrade before you splurge on a new preamp in case you’re using the stock JJs in RP-1.