Samuelg,
I hope some of the suggestions above have been helpful. It is a bit hard to give focused advice, given that we don't know some basics, like a price range for instance, but, you are probably looking for a wide range of options. For example, I like the suggestions of the Devore speakers, but, I have no idea if they are in your range as a choice.
I know you mentioned that you have the Harbeth 40.1s, so you are not looking at Harbeths, but, I think that their other models sound substantially different from that speaker and the other models are MUCH easier to drive than the 40.1s (these are particularly demanding speakers that can sound dead or flabby with the wrong amplifier). Perhaps the 30.2s would be a candidate.
I again mention the Audio Note speakers. It is interesting that you have ProAc and Harbeth speakers, because one of my favorite dealers has ProAc, Harbeth and Audio Note as his primary speaker lines. He also has Nola, although for some reason, they are not that popular (the cheaper models sound good for the money to me). This dealer, DejA Vu Audio (Northern Virginia) only sells tube amplification, so their speaker preferences are matched to this type of amplifier. Most often, when someone comes in looking at all options (i.e., they did not come in looking at just one specific model), they end up with an Audio Note speaker.
If you can stretch a bit in price, Charney Audio makes terrific speakers that are in the 99 db/watt and higher level of efficiency and they are fantastic as long as one is not interested in extremely deep bass.
