I really tried to love the Mesa 400+, changing pre tubes, speaker cabs, EQ settings. I just couldn't get the baked in mid range out-- or should I say, lack of lows in.
Did you try the 12AU7 P/I conversion? Search this forum, PBG has written good 'papers' on Mesa amps and is the best resource I can think of when it comes to this particular amp.
The guy said it has TAD KT88 tubes in it. I think they were swapped out. I think Reeves shipps with JJs. It was bought in 2015 so maybe that's what was around then?? Don't know.
Not much to choose from anymore, in bass amps or HI-FI I'd recommend SED KT88's (the real Winged 'C'/Svetlanas from St.Petersburg, Not the Svetlana/SED's sold in the US by New Sensor Corp.) but they are out of production since 2012. And now crazy expensive. I have distribution rights for them, but cannot get them anymore - the last stock of these were sold to a well known HI-FI manufacturer. They are really good, last long, can take high anode/screen voltage, and are capable of producing really good bass - tight all the way down. I'd rather put in used SED's than new chinese/slovakian in a demanding application like you describe.
That said, JJ can sometimes sound good, any tube can. But of all tubes today, JJ's are most prone to rapid breakdown. I'm speaking now of their power tubes; their preamp tubes are good if you find non microphonic ones and like their sound.
TAD is not a favourite either, they use the same chinese KT88's as everybody - but there are many different ways of selecting tubes.
I think quantity goes first in the TAD camp, but you can certainly find good TAD's and hope they last.
Get current production Shuguang's from a trusted rebrander/selection with warranty, bias safe, and you're good for a few years.
I'm obviously biased
when it comes to tubes, but try not to promote my brands on this nice forum, but if you need more specific
brand/rebrander info/advice you can always, uh.. PM me