Joule LA100 MKIII or Granite Audio 770FP or Modwrt

I've never heard a loss of dynamics running a passive preamp in my system. If your source has low output impedance, at least 2V output, and you're running short interconnects you should be fine since your amp is passive friendly. Furthermore, you can avoid most impedance matching issues that cause loss of dynamics by using a transformer/autoformer based passive (like the Promitheus) versus one that uses resistors. However, I own one of each type and again have had no issues with loss of dynamics.

I had the Modwright and the Joule. Two different sounds. The Joule was more tubey and the Modwright was neutral. The Modwright was more refined and faster though. I ended up keeping the Modwright at the time. The Joule is much more plump in the midbass (too much for my liking). The Modwright has more gain than the Joule also. I now own a Supratek, which is comparable quality wise to the Modwright, bigger sound (6SN7s) not as dynamic as the Modwright just as detailed and warmer. The Modwright did soundstage better. I'd go with the Modwright given the rest of your equipment. Get one with a tubed rectifier, suppose to make it warmer, and possibly the new caps, more refined.

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.