For classic rock tones especially, I would go with Greenback, rather than a V30.
But what about the speaker rating? I always thought the conventional wisdom was that a lower-rated speaker (e.g. a 25 watt Greeback) would break up better and presumably sound better at lower volumes compared to a higher-rated speaker (like the 60 watt Vintage 30).
With a TT on 7 watts, you won't be getting a greenback to break up anyway.
Personally, it has not been my experience that lower power rated speakers sound better at lower volumes. Remember that the power rating is a MAX power rating, not a suggested amp power match.
there are so many factors at work here. A big one being whether you like to get speaker break up in as part of your tone. Personally, I don't like it. In all honesty, there isn't a guitar speaker around that will give you that with a TT, except perhaps a blue.
In the end, I would go for the speaker with the right frequency response for you. It might be rated at 25W, 65 W or even 100W. For example, I've heard V30's sound superb with the right amp, at very low volumes. Personally, I use a 30W amp, set at half power, through two 65W speakers. These are just killer speakers with this amp.
So, basically, my advice is to ignore the internet folklore that says that lower power speakers sound better at low volumes than higher power ones, because it simply is not true. there are far too many other factors at work.