they are essentially the old scumback speaker... intended to duplicate or emulate the old pre rola british speakers (think greenback). They will have a midrange hump and break up nicely. Not at all an american fender sparkly tone, but lot's of guys like where british speakers meet fender (mid scooped) amps.
I just got a pair to use with my Marshall, so I can't really speak for the Twin Reverb. But they sound very close to the M75's in my other cab, that are supposed to be pre-Rola Greenback's. The M75's are a little darker and smoother, having less highs. I'm very impressed by the Legacy's. If you need to approximate how they sound in a Twin, I would do a search on how Greenback's sound in a Twin instead.
So, I have two Legacies. Both are large H dustcap, and 30 (actually 35) oz. magnet. One is the 75hz cone, and 30W, the other the 55hz, and 65W. I've used them separately, open and close, in a pine cab, and together, in a pine 2x12.
I do not like the match in the 2x12. That much I know. I haven't had the chance to experiment further. I'm sure there's a pair that would rock, but all I know right now is I don't like these two together.
I do very much like the 55hz one by itself (and to a somewhat less extent, the 75hz, which is very similar), in an open backed cab. It sounds nice and full, with a sweet top end. I think the way they break up is really very well suited to a clean Blackface tone. Speaker breakup gets a bad name. It isn't really at all like tube breakup, yet folks hear that word "breakup" and think that means it's not clean. You can have both. IMO, that breakup lends a really alive feeling, sort of like the traditional American style speakers, but nowhere near as harsh or aggressive.
Remember though, I do have the large H dustcap. I like a smoother top end. I think the standard Greenback would be significantly smoother than a JBL or clone, and then mine would be yet smoother.
In summary, I like the one Legacy because it sounds big, and full, and has a fairly even EQ. It feels alive, and present, without being overly aggressive, and despite an overall warmth. It has a smooth top end, that can still sound bright, without getting sharp. Inneficiency is a bonus.
I would like to try a bigger magnet. I also thing I prefer the higher wattage to the lower. That's all fine tuning though. In general, I think the combo works very well, even if I don't yet have the ideal setup.
In my Vibrolux Reverb I have a 10" Legacy, and a 10" Gold. They sound gorgeous together. The Gold adds more chimey top end than the Legacy, but not as much as the stock speakers, and the Legacy keeps the top end sounding slightly rounded and smooth. The mids are emphasized more than the stock speakers, but that's what I was looking for. The mids it has are also much more pleasing to the ear being smoother yet still focused...possibly due to the Gold it is paired with. The low end is of course tighter as well...even the Gold has a tighter low end than the stock speakers, but the Legacy is tighter than the Gold and tightness is very important to me in an amp.
One downside is that the cleans no longer really sound like a blackface Fender amp, nor does the breakup sound like typical blackface Fender breakup (but that's what I was trying to get away from). Still the cleans sound gorgeous in their way, think JTM45 mixed with a Vibrolux, and it's similar to that...but still the tonality is skewed more towards Fender sound.