Joule LA100 MKIII or Granite Audio 770FP or Modwrt

Re the Baron, a few observations I have from operating my earlier model which used 5881's exclusively. It benefitted from a cap mod done by Mesa, using Wima Caps, but that is as far as I went.

The gain issue, and the sound as well, seems infinitly variable, with selection of the 'imaging mode' and feedback. The all pentode mode is very high gain and will create some loss of flexibility using a volume control, unless you have very low efficiency speakers, with out regard to whether or not you are using a passive or active pre-amp.

FWIW, in my experience anyway, the using a passive pre-amp has more to do with reducing the 'dynamics' often introduced by an active pre-amp, not necessarily to help match gain. I have the same gain issues with a line stage with a 12db gain as I do with a pre-amp with a 26db line stage gain (An ARC SP10, a Primaluna 3, and an EAR).

Re tubes....I've only used 5881's in mine. I tried some 6l6GC's but it wouldn't bias properly and I didn't want to have Mesa reset the biasing parameters. But, I have used, in other amps, the Tesla (Gold Aero badged) E34L's, NOS Tesla EL34's, and found them both on the bright side of neutral. Great clarity however, a nice crystal clear sound. IME (as well as that of others I think) the Baron doesn't need any help in the high frequencies. To the contrary, a little damping might be advantageous.

Based on my experience using these tubes in other amps, and my experience using the Baron, if I had the multiple type selection that you have I'd be very interested in trying out SED EL34's my favorite new production EL34, both because of its neutral sonics and its ruggedness as well.

yeah, the tube rectifier in the Modwright is worthwhile!

I beg to defer re "Tubey" since for me it is always associated with rounding and smoothing the sound, usually at the expense of detail retrieval and speed.
I find the LA-100 Mk III rich sounding as opposed to plain "Tubey".
It is like adding a little bit of cream to a clear broth. Not too much to make it thick but enough to make it beefy and rich.
I would not call it romantic sounding:
It is more like comparing an anorexic girl to a voluptuous girl (as opposed to fat).
Here are some of my impressions:
The LA-100 Mk III attack is second to non and so is its mid range fullness and bass. Its clarity is excellent as long as good 5751 tube is being used. It is slightly mid-range forward compared to the LA-150 MK II and slightly less ambient and airy (it feels more like seating in the first or second row as opposed to the 15th row) but than a little more "beefy" than the LA-150 Mk II to my ears.
All these impressions are in my system and my room.

So to summarize: Rich - yes. Tubey - depending on your definition of tubey.

Have not heard the Modwright so cannot comment.

I had the Joule for several years and will agree 100% that it is a fine sounding pre....and also that it needs the right power cord to "wake" it up. I used a TGA Audio SLVR on it and the difference was not subtle.

The Joule with a good PC doesn't have a rolled off sound at all and is dynamic, with excellent detail, but it is definitely slightly romantic - I used to call it making the music more beautiful then real life. But the soundstaging - layers and layers, deep and wide - and the delicacy of the highs, the air around the instruments, the natural vocals and timbres et al...that sound can be very addictive. Joule will also work with you to fine tune/custom set it to match your system - they are great people.

I eventually replaced it with the Herron VTSP-1a which was a fabulous preamp (and slightly underrated IMO). That piece combined the things I loved about the Joule with a bit more neutrality and still stronger dynamics, plus it was a match made in heaven with my Herron M150 monos. I LOVE Keith Herron's gear - it just took my system to the next level. But I could've happily kept the Joule forever.

Have Baron with the Modwrightd. The combo is very good.
The Baron is really good with EL34s.