Well I got the amp back friday so I'll tell you what I've found.
First off, Twins are heavy beasts of amps, and now this thing probably weighs 15 more pounds due to the oversized magnets in the Celestions. So that's a negative.
I am told that these speakers are gonna take about 100 hours of playing through at fairly loud volumes before they break in completely and are at their optimum performance. Which is actually exciting because they already sound GREAT!
The first thing I immediately noticed is that these speakers are loads louder then the stock fender speakers so I almost blew my head off when I first plugged in my guitar and set the volume at 4, where I normally set it. For playing out live this is good news, but is bad news for my bedroom jamming. The first thing I noticed was how different the harmonics and overtones sounded in the amp, the whole voicing of the amp is different, it still sounds like a twin, but it has much more warmth then I'm used to hearing.
The next thing I noticed was the significant increase in low end response and the bass frequencies are much much tighter and less flabby sounding then before. The overall voicing of these speakers is darker and I found myself lowering the bass knob down considerably from where I normally have it. I'm not someone who normally plays with a lot of midrange, mids sometimes sound a little harsh to my ears, but on these speakers the low end mids sound absolutely beautiful so I found myself dialing in a lot more mids then usual as well. The high end has a lot different texture to the sound and actually has a larger range of tones then the stock speakers did. So they can actually get a lot more ice-pickey then the stock speakers then the originals if they are raised to high, but dialing them down a little took care of that problem. This is not too say that the speakers are brighter or more harsh in the high end then the old speakers, they just have more high end range then the old ones, so you have to dial them down a little more then the stock speakers to mellow them out.
Playing my Jap Jag with Seymour Duncan Antiquities with amp highs set to about 6, mids at 7.5 and bass at 7.5 with reverb at 2 sounds absolutely great. Very rich warm tone, and if I roll the tone and volume down on my jag to just the right point I get a great warm attack from my guitar's pickups and a large snappy bass tone if I hit the strings hard that doesn't ever sound harsh, just more aggressive.
My old amp settings used to be highs at 7, mids at 5, and bass at 9. Now my settings are totally different. The mids on this speaker combination is what sounds really great to my ears so I end up using more mids then I've ever used before.
I've tried all my effects through this thing and with fuzz or tubescreamer type pedals this amp sounds great. The harmonics on this amp are super warm and rich and drive pedals really help bring this out.
So if you've got the money I say mod your vintage reissue!