This has been a question of great interest to me, as an owner of the M-30's who is thrilled with the mid's, but wouldn't mind a bit more warmth. (This is a question that has come up before on this forum by the way, but without much response.)
So, let me report first that I recently found a very good comparison of the two speakers contained in a review of the Monitor 30's in Stereo Times from January 07. It's not the easiest review to find for some reason...I think the easiest way to find it is to look for the directory of Stereo Times on-line and go from there. The very interesting conclusion of the reviewer is, in short, that the M-30's are notably less forgiving than the HL5's (and the smaller P3's): "While the [HL5's] also revealed the limitations of gear [and source material], they tended to work around the flaws and to maximize the gear's good points. Not so the 30's: flaws were never glossed over." I found this particularly interesting because, after nearly a year with the M30's, I have come to a similar conclusion--as good as they are, they don't let gear or source material flaws off the hook at all--which I find a bit frustrating, having turned to the Harbeths in search of a more forgiving speaker than my previous speakers. (That having been said, they are the most forgiving speaker I've used on a long-term basis in years.)
Next, I had a talk with a dealer that knew both speakers not long ago. He described the HL5's as being more "voluptuous and warm" than the M30's. However, he described the M30's highs as being more detailed and nuanced than the HL5's. Yet the HL5's were not at all bright, according to this source. I do not find those descriptions particularly inconsistent with the Stereo Times review.
There is no place convenient for me to hear and compare both speakers. Consequently, I've been looking for opinions on this issue, with the expectation that I would buy some HL5's on Audiogon, or otherwise by mail, if the reports appeared to lean in their direction. The reports mentioned above came to my attention only in the last month or two. I'm curious to hear any other reports, and hope these are of some use. In the meantime, I'm saving my nickels, and will likely take the risk with some HL5's in the next couple of months...if I think I can fly under the spousal radar screen tolerably well...
(I should add that I am under the impression that in fairly small rooms the HL5's can sound bloated in the bass. My room is 14x20, and I don't think I will have that problem. As of yesterday, I am trying out a subwoofer also, to see if that gives me what I'm looking for. I think, so far, that it is nice, but not likely the answer. I don't want mere bass...I want a big warm bath of sound!)
Any other thoughts from anyone?