I've had a Bloc 40 since they first came out. I've modded mine to 80 watts. The Block 80 and the Bloc 40 have the same circuit, but the 80 has a higher voltage power transformer, and I put one in my little 40, as well as an extension speaker jack.
Your limitation will be the 4 ohm speaker. MCM has a couple, but the high end is limited. I have an Eminence 4 ohm kappa pro, but i've never used it alone as again, not enough top end for me.
A 4 ohm Peavey Black Widow might be nice. An Eminence Legend would be close to the original speaker. http://www.usspeaker.com/Legend%20CA15-4-1.htm
How much do you want to spend?