Well, this will get you started on finding what your amp is all about.

http://www.ceriatone.com/Manuals/ceriatone%20hrm%2050w%20100w%20manual.pdf

One of the main differences between the HRM and OTS is that it has a second tonestack for the OD channel. So when you have just the OD without PAB engaged you're going through both clean and OD tonestacks. With PAB engaged the clean tonestack is removed. This essentially will give more gain since the load of the first tonestack is gone. Also, depending how the internal HRM trimmers are set the tone may change. This can all be adjusted to taste.

Other differences are no snubbers on V2 and either no LNFB or switchable LNFB on V1. There are also slight differences to the OD portion of the circuit, like a internal trimmer for the OD entrance. Most of this gives the ability to really tweak the heck of the this amp.

I have a HRM 50 and really love it. I just had someone over yesterday and was tweaking the knobs while he was playing a standard Strat and was getting some great tones out of it. It's the first time that someone else was playing and I was just sitting listening. It takes a little while to get used to the amp. He was a little intimitaded at how to set it up. Thus me doing it for him.  Best amp I ever owned. And I've owned a few.