Thanks for weighing in - I was hoping you would... I'm leaning towards a set of JJ KT88s from eurotubes, and having my amp guy bias them properly - although eurotubes state that their tube sets are a direct match for my amp, and no re-biasing is necessary... Any thoughts on that???
Mesa's tube bass amps ARE basically slightly revoiced guitar amps from the ground up and it's easy enough to figure out what tube current disappations correlate to Mesa's "rating" system, and Mesa runs their power tubes very cold but does not drive them with power, but rather cascading voltage gain, hence the massive power distortion and bright sound. The best thing to do with the rating system is to ignore it. Tubes change in current output over their lifetime and many good tubes have been trashed that needed only a simple bias adjustment.
The preamp's coupling caps can be changed for better low end, (Mesa's preamp begins rolling off bass frequencies very steeply around 80 Hz - because they are guitar preamps.) but it ends up re-voicing the entire amp and changing all the tone controls and the end product is very "Fendery" which is where the circuit started out anyway. Have your tech install bias pots and bias the amp for maximum stable clean output. A good tech could add a negative feedback loop to the output transformer to get even more clean power out of it, but it's going to seriously alter the sound, much in the same way that the 12AU7 driver tube swap does. One positive to this however, is that if you DO go this route, it's an easy and completely reversible mod and using the 12AU7 will help to get the most out of the power section if you do. 12AU7's have a much higher plate current capacity than 12AX7's. That translates into more clean power to drive the output section. These are all things that will make the amp have much more punch and a better low end, but it will change the tonal character of the amp. IMO, that change is for the better, but lots of other folks feel otherwise.
I was already planning on trying the 12AU7 driver tube swap... Also, agreed that my 400 is quite bright - rolling off some highs will not be a bad thing for me... While the 400 isn't quite as powerful as an SVT-II(my other tube amp choice), it's much lighter - and I tend to prefer the Mesa sound over the Ampeg sound - so far, I'm fairly happy with my purchase...
Note that the changes I am recommending are going to make the amp sound more like the Ampeg, as it has more clean power on tap and was designed that way. With the 12AU7, the preamp voicing will be the same, complete with all the over-the-top gain, but the passive EQ center frequencies will change. The graphic EQ is separate s/s circuit and will be unaffected. The Master Volume knob will become much more useable, and the Volume controls will lose much of their "fuzz boxy-ness" and actually perform as advertised.
Just to give you an idea of what to expect, there's absolutely no harm in swapping back and forth between the 12AX7 driver and the 12AU7 without doing anything else to the amp, so you can literally A/B that part of the mod yourself. Let me know what you think about the changes in sound if you decide to try it. From what I remember on Talkbass before I was banned, everyone who tried the mod loved it, and then after I was banned, several "expert" critics emerged from the woodwork to 'refute' me. I doubt most, if any of them, even had a Mesa tube amp.
My Mesa still sounds like a Mesa, but it has a bit more punch. However, the light weight is because the amps use relatively small power and output transformers and the amp will never be more than a 225 watt amp; it will just be a cleaner 225 watt amp with better lows. I think my preference for setting up my tube bass amps for the maximum potential is why my views on tube bass sound conflict with so many other folks' ideas; they're used to buzzy overdrive from cold tubes being driven with insufficient power. There IS a "danger" to what I do: you can't get away with using crap tubes, but aside from that, the only thing you will notice is a slightly physically hotter amp and power tube life of a couple fewer decades, so you may end up needing to change them if you gig every day...in about ten to twenty years.