Here's a few differences between the older Mark I vs. the Mark II series from what I can remember...
More power - 700 watts into 4 ohms compared to 540. this provides an increase in headroom over the Mark I series.
Low end is said to be fuller/fatter and extends to lower frequencies compared to the older model.
The EQ section has been reworked... the highs and upper mids sound more refined and hi-fi, but not as sterile as the Mark I. The highs of the older model were sometimes brittle and less refined sounding, depending on settings.
Cosmetically, the Mark I amps looked a bit different... a bit utilitarian IMO. The Mark II series has a sleeker look.
There was also an issue with a number of the Mark I amps where there would be a power spike causing a loud "thump" sent through the speakers upon turning the amp on or off.
I own a 1001RB-II, but I've never tried the older Mark I series, so the above comments are from what I've heard from others. Supposedly, these tonal differences are somewhat subtle and are not what one would call a "night and day type of difference"; but they are noticable to the discerning listener.
I did find an article about the classic "Amp Thump" that plagued some Mark I owners...
Link Removed
- Ugly.