Jonathan,
While I have no experience with that particular amp (i.e., Blockhead), one word of advice: play EVERYTHING before you buy.
Marshall reissues are still not handmade, hardwired like the originals were. The cabinets aren't as sturdy as before, and they ain't made in ole' Blighty (Brittain) like they once were (fabbed in the far east, chassis placed in the cab in and Union Jack emblem applied in England, LOL!).
There are so many 'boutique' amp guys out there, and they all make hand-made hard-wired stuff. The main variables (besides aesthetics/cosmetics) are how close you want it to sound like an original JCM versus how much added flexibility you want, i.e., multi-channels, footswitching, effects loop, additional gain, etc.
Greg Germino's amp line is as about as close to that vintage Marshall plexi sound that you can get, with top-shelf components and construction, but no additional channels, features, gain, etc. are available. There are lots of others. Harry Straub is one of them (that's what I bought), but he only offers a high-gain mode to supplement the standard plexi circuit (additional channels, reverb, effects loops, footswitching need not apply here, LOL!). Victoria, TopHat, Bogner, Modern Vintage, etc., all have good Marshall-style clones. Scott Splawn can take your JCM-2000 and modify it so that you can have added gain, features, and flexibility.
Have you also considered a vintage Marshall? They're pricey, usually beat to sh*t and usually need considerable maintenance and/or updating. They can also be modified by guys like Splawn or Bradshaw to increase flexibility and gain.
Sorry I really didn't answer your question, but I figured I'd offer my two-cents worth here. Food for thought, at any rate.
After looking at all the options, I ended up with an heirloom-quality amp rig, all in gorgeous hardwoods, for MUCH less than a Vintage plexi (and that's before repairs and mods) and most of the Tolex-covered plywood boutique stuff (like Marshall USED TO make). The more I play that amp, the more the sound blooms and the more I fall in love with it. If you want vintage plexi tones, with hot-rodded hi-gain Marshall available, as well as, with sweet hi-headroom cleans no vintage Marshall can even dream of, you could do much worse than a Straub amp. Ck out my thread for pix and info:
http://club.alembic.com/Images/449/16159.html?1112366713 Good luck on yor quest.
Cheers,
Kevin
