It doesn't have to be in the tubes. Mesa rectifiers aren't exactly known for their tight bottom or subtle lows and the 3 channel duals and triples have an even sloppier bass end than the 2 channel amps.
Also, the cab plays an important role in the sound. I found the oversized rectifier 4x12" cabs to be WAY too bass-heavy to actually go with a rectifier head, especially the current production.
That said, the power tubes have a bigger impact on your tone than the pre-amp tubes. I suggest you contact Bob from Eurotubes after you read this page of his:
http://www.eurotubes.com/euro-n.htm
Bob has helped a lot of my Mesa cranking friends to get their Mesas to sound as great they possibly can, without any need to modify the amp itself.