I've got a Mark V and an Express 5:25Plus (and a Royal Atlantic RA-100). I've also owned every Mark series back to the IIB, and most of their other series amps over my 30 year relationship with Mesa. The Express "series" is not in the Mark family. It isn't intended to be part of the Mark series. It was designed to be voiced differently, to hit a different part of the market and to hit a different sonic palette than the Mark palette. Could you spend time tweaking and trying to get the Express to sound like some of the sounds in the Mark V? of course you could. There will be some overlap and some commonalities between the two as they are both primarily "american voiced" amps by Mesa, so they have some common lineage.
The Express stands on its own, with its own strengths, weaknesses, features and limitations, and is a really nice amp, without comparing it to a Mark V, Royal or Rectifier. I would say you have a really good two channel amp with a ton of tones in it. Get the most out of what you have and find the tones that work for you in your amp, rather than worry about what you don't have or what it doesn't do. You could spend hours comparing its limitations to the MV, the Rectifier, the Lonestar, and on and on. It is its own amp- enjoy it and grab as much tone out of it that fits your style and your ears. If you can't get the tone you want/need out of it, then look to figure out what amp will get THE tone for you.
Good luck and enjoy the ride.
Brent