I use an Avatar 2X12 cab (among others). Currently using it to record my band's CD (out of my collection of Carvin, Mesa, & Pacific cabs). I have the semi open back (oval opening) loaded with a G12H30 & a Vintage 30. Kickass cab! HIGHLY RECOMMENDED!
Actually I must add that I have heard very good things about avatar cabs...
But since you have a JCM 2203, may I suggest Classic Leads (not the cheapo ones... avatar cabs offer options for both!). I just put 4 new Classic Leads into my 4x12 cabinet and they just sound so damn good with my 900 SL-X. They sound awesome with 800s too!
Lots of smooth midrange, good lows and highs too, and despite being higher wattage then the 75s they are warmer at the same time!