Don't know if my experience can help, but I have both the P3ESR and the M30.1s in my house. My primary listening room is 12x17x8, so a bit smaller than yours. I bought the P3ESR first and enjoyed them a lot in my mostly Naim system. I then bought the 30.1s, hoping for more resolution and scale and that is pretty much what they provide.
The first thing I noticed when I replaced the P3s with the M30.1s was that I had to lower the volume. The 30.1s are easier to drive. The difference isn't huge, but noticeable. They are more efficient, which I guess makes sense - the sealed vs ported thing.
The next thing I noticed was that resolution went up by a noticeable degree. The tweeter in the 30.1 is more refined. The P3 is no slouch, but the resolution of the 30.1 incrementally extends everything the tweeter does - soundstage depth is deeper, there is more observable detail, initial transients are a shade more natural sounding and there is a sensation that harmonic information is just more saturated, so to speak. I would think this would sound terrific with your tubes.
The third thing I noticed - again a bit of the sealed vs ported thing - was the box. The punchy bass of the P3ESR was replaced by a less punchy, less taught, but deeper bass - at least initially. I say initially because I had originally situated the 30.1s roughly where I had the P3s, about 5 feet from the back wall and 4 feet from the sidewalls. As I moved the 30.1s closer to the sidewalls I found I could tune the mid bass punch. Makes sense, they are bigger boxes and they react with the room more. When I got them on some solid open stands that improved again. The 30.1s respond to placement a lot more than the P3s. This can become a problem if you have to put them on shelves or don't have the freedom of moving them about the room, so that's a consideration. Your room is bigger than mine, so I would think anything in the Harbeth line is fair game for you if you have the liberty to move them about. The bass in my room from the 30.1s runs down to about 60Hz I would say. I cross my JL subs over at 50Hz and find that the subs and the 30.1s seem to cross over and blend just right that way. The tonality of the 30.1s I found blended seamlessly with my JL110s. I can't tell where one ends and the other begins other than the frequency. The P3s made useable bass down to around 75Hz in my room, so I crossed the subs over at about 65-70Hz for them but I could never get as good a blend with them as I can with the 30.1s. In both cases the subs are directional because they are cut over higher, but with the P3s, it may have been that the JLs were simply putting out a lot more of the bass content of most of the music I play, so I became more aware of them.
The 30.1s also seem to respond to power, which is in perfect keeping with what Shaw says. I think your Quicksilvers are plenty, as is my Naim XS and my Naim SN2, but the bass in these speakers changed in just the step from the XS to the SN2, so I have a suspicion that these speakers can be more dynamic than people think if they get enough power. I have not dipped my toe in those waters yet, but I'd love to hear what a brute could do with these.
Lastly, the 30.1s simply sound bigger. Duh, right? The volume of air it moves is simply larger and in my smallish room you notice that pretty readily. What I didn't expect was that the 30.1 can fill the room rapidly as volume goes up and when playing loud can pound the room pretty hard. As I raise the volume on the P3s, it certainly got louder and they aren't wallflowers, but I could tell where the energy was coming from if that makes sense. The sonic images scaled to a point and then just got more intense. With the 30.1, when you turn them up, the sonic images seem to get larger and louder together to a greater degree than the P3s and with more harmonic overtone evident. They are simply more enveloping.
I still love the P3s, so I hung on to them to use them in another room, but I have to say, I haven't been tempted to put them back in the listening room. I feel like I've only scratched the surface of the 30.1's potential. Makes me really curious to find a used pair of the 40.1s : )
I think the advice to pick up a used pair of 30.1s or even SHL5 pluses (might be closer to large P3s in nature than the 30.1) and try them out would be the right approach for you if you could swing it. I was where you are. I knew I loved the P3s, just wondered if more would really be more or just more money. I figured I could swing buying the 30.1s and then just selling them again if it didn't work out. That gave me a few months to suss out a feeling for each before arriving at my not so responsible decision to keep 'em both!