@jfmusic I know the AHB2 really well. I owned that amp so many times. In both stereo and mono form. Your warm Harbeth will go nicely with the ultra-clean sound of the AHB2. Now that sound is not for everyone, but I like it. Some people find it sterile.
The AHB2 monos are rated at 6 Ohm, not at 2 Ohm like the stereo AHB2. The mono AHB2 becomes quieter and louder than the stereo AHB2. Otherwise, the sound is the same. The power delivery is what is different. Sometimes the stereo is better than the monos and vica versa. It depends on your speaker. For the Harbeth, monos would be better. The AHB2 does suffer some of the same things as other stereo amps going into mono, but it is a bit more capable in this regard. You may want to ask Rory Rail at Benchmark for their reasons why they like monos (of course they sell more too).
I have a speaker somewhat like your Harbeth, the Yamaha NS5000. It is also a 6 Ohm speaker. The single AHB2 drives it well, though monos will provide greater range in the volume. I did not like mono in my old 2 Ohm Thiel CS3.7, preferred the stereo AHB2. That did not have much power in stereo, so I gave up on the AHB2 for that setup.
I came back to the AHB2 with the NS5000. I did a shootout with the AHB2 stereo | Sanders Magtech | CODA #16. I preferred the Sanders and #16 over the AHB2, mainly because of the greater power.
The AHB2 was also a tiny bit harder on top with the Zylon drivers of the NS5000. Even though I think the NS5000 is slightly warm I think a warmer amp than the AHB2 is needed with the NS5000 for my ears, but not too warm.
If you are interested, I have a 1-year-old silver AHB2 sitting idle that I can sell. I only expect to use it again in about 2 years when RAAL 1995 comes out with the replacement for the SR1a earphone.
BTW - I use Audience FrontRow cables with the AHB2 and all speakers, but the Benchmark speaker cables with SpeakON termination worked well with a bright headphone I owned.