I think most Marshalls are like that, more treble than low end. Mesa's have bottom but I think they are muddy.
Might look into the possibility of Changing the tubes to a 6550 or KT88 if that is an option.
Those tubes tend to add more low end.
One nice tool is the Bellari 562 Sonic Exciter.
Its like a BBE but Tube Driven by a 12ax7.
I am not much of a BBE guy but I love my Bellari.
I recorded a Marshall JCM2000 TSL100 with a BBE 482 2 weeks ago and then the guy used my Peavey Triple XXX with the Bellari and he wanted to buy both my Triple XXX and the Bellari off of me.....
They were about $370 new though, kinda pricey.
Might be happier getting the most money out of your Marshall Head and getting a used
Peavey 6505+ or a 5150II.
I bought 1 Marshall JCM amp and returned it for an Original Peavey 5150 and I have never gone back to Marshall.
Tryed Mesa....
Poop......... loud, but too muddy
My buddy kept playing head games, started with a Randall Cyclone, then a Krankenstien, then
a Randall V2, and finally ended on a 6505+.
You can get a Madison Divinity pretty cheap.
www.madisonamps.com
100 Watt All tube, sounds like a cross between a Marshall and a Mesa.
Much better than a Marshall and close enough to a Peavey for Rythm tone.
You can mess with the tubes, but in the end, its probably inevitable that you will want another head.