If you can't audition, trying to buy a speaker on other people's opinions can be dicey. There are a number of reviews and opinions about how the speakers sound to other people. If they don't give you the confidence to take a plunge, I can think of two options.
If you're going to buy new speakers, contact a Harbeth dealer who appears to carry the full line and see if you can have either the M30 or the SHL5 sent on approval for an in-home demo. If the model you order doesn't work for you, you can return it and try the other. OTOH, if you're planning to buy used, buy either model and see how you like it. If you are not satisfied, you can resell it here on Audiogon probably without any substantial loss.
I own the M30's myself. I play a lot of small group and solo jazz, female vocals of all sorts, and small group and solo classical. I really enjoy the speakers.
Good luck.
Jim