The Tannoy SRM15X are a large set of monitors that I have used strictly for listening back on. I haven't used these speakers in a studio setting exactly in terms of monitoring or mixing - only as another option to listen to mixes back on. I believe that these speakers require a separate power amplifier, but I'm not sure as the ones that I have used were already set up and I didn't have to play around with them at all to get them to work or to control the volume or anything like that. While I haven't used these for mixing or monitoring/play back during recording, they do have a great sound and I would imagine that they would be suitable for both.
The sound of the Tannoy SRM15Xs are extremely full bodied. These speakers are huge and cover the entire frequency spectrum very well, including the elusive low end. Like I said, I haven't used these for mixing, but since they do cover a wide range of frequencies I don't see a reason why they couldn't be used for mixing, and if not at least for monitoring during recording. I do find that they have a bit too much mid range presence, but this is minor thing as long as you tune these to your room if you are planning to use them for mixing.
I've only used the Tannoy SRM15X a small handful of times and hope in the future that I am able to use these for a wider variety of applications because they are a great sounding set of monitors. It is hard to really get a feel to see if these monitors will be a good set for mixing until you actually try it and listen back on other speakers, so I can't say I would recommend them for mixing but it is very possible that they would be great for this application. I can say that if you are looking for a full sounding set of speakers for a home audio set up, that the Tannoy SRM15Xs would be worth looking into.