I think speaker size is no longer an indication of tone. Probably was 30 years ago. These things do rumble though but I think that was designed intentionally, regardless of speaker size. Anyway, this head has a HPF witch cleans up as much of that rumble as you want. That and sweepable high and low mids makes this thing pretty versatile.I would think that quite a dialable rig. Nothing "wrong" with a coupla of 15"s - you got the punch (maybe not total spank but it IS bass ).
Mesa is good stuff, you should be all set. Congrats - cool.