Tube Rolling Advice for the Backert Labs Rhumba Extreme?

Tomorrow I have a Backert Labs Rhumba Extreme 1.2 being delivered, which has been upgraded by Backert to 1.3 specs. It will be driving the Luxman MQ-88uSE and the front end is a Qutest with M Scaler (Innuos Zen MK III server/streamer). Speakers are La Scala IIs, placed about 15' apart in the corners at a 45º angle. Soundstage is excellent and expansive with remarkable placement of instruments across the sound field.

If all goes well it will be replacing my L-509X, which is currently only being used as a preamp for the MQ-88uSE, so it’s being underserved in my system and will need to find a better home.

I am interested in any advice about tube rolling with the Backert Labs pres. With the Chord digital front end, I am getting a very layered, dimensional, detailed sound and thanks to the Qutest filters I’m able to adjust filters to best match the amplification. The MQ-88uSE has a gorgeous, full, warm sound and right now the 509X’s solid state pre is giving a nice sharp, detailed, and dynamic profile to sharpen up the tube sound a bit.

I don’t want a syrupy or bloated tube sound, more airy and detailed on the top with holographic imaging with solid heft on the bottom end. I’ve replaced the preamp tube in my Cronos Magnum II with a Cifte and was really happy with the impact on the sound.

The Backert comes stock with new Mullards, which I expect may have a slightly warmer sound than what I’m chasing.

Any advice?

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The Backert is getting its first workout tonight. I'm not ready to make any gushing endorsements just yet but I am very impressed, particularly with the imaging and separation. I've noticed it's a little "soft" sounding - not syrupy or bloated or muffled, just a little polite, particularly on the bottom end. I suspect it may be the Mullards.
Call Andy at Vintage Tube Services and tell him your gear and what sort of sound you are looking for and see what he says. Something as simple and reasonable as a pair of RCA Cleartops or Raytheon Black Plates like the ones used by Baldwin and Conn might get you where you want to go without spending a fortune.  But I wouldn't do anything until you get about 150 hours on it.  

I find most tube gear is soft on the bottom end before running in ~80 hrs.  At that point the bass tends to tighten up in my experience.

I've noticed it's a little "soft" sounding - not syrupy or bloated or muffled, just a little polite, particularly on the bottom end.
Thanks everyone... so it seems like there is a fair amount of burn in time required for these caps then. This could explain why I was a little surprised by what I was hearing with the "softness" in the bass, versus so many of the online reviews and marketing material that say if anything the bass is a standout feature of these preamps.