As it happens, I have experience of PMCs and Harbeths in a room almost exactly the same size as yours.
I've been using 7ES3s for some years, but 'just for a change' bought a pair of PMC GB1is to try. Like you, I noticed the punchy bass, but felt that at times they excited the room too much for my comfort. Mids and highs were very good, but not quite in the same class as the 7ES3s.
Just for an experiment, I tried the GB1is in my main system and they were a revelation, easily filling a larger room.
I get useable bass down to 31.5Hz with DRC from my 7ES3s, so I'm a bit baffled by your report of lack of bass, even from the SHL5+s. (I've also had them in my second system but had to part with them because they were too big for the room.) As I've said before, I feel that if you are prepared to invest in a little more power (say at least 100 wpc) then you will get much more solidity from Harbeths.
Based on my experience, the challenge you are faced with is getting the level of performance you're looking for from a such a small room. It's very easy to over-excite a small room (even with DRC) and you then get a very blurred, muddled presentation. You basically have to compromise. I won't say anything against PMCs as they are a very well-respected brand. However, their 'house sound' does seem to be on the forward side and I would be careful of that in a smaller space. The more relaxed presentation of the 7ES3s suits my room and IMO provides a less fatiguing experience in the longer term.
But everyone is different of course. Good luck!