I purchased a used MAG300 about 5 years ago when I needed more oomph. I still use it regularly, and it still powers my 4x10 with enough juice to keep me heard about a loud drummer.
The EQ is great, and the low and high boosts are a welcome addition when you need 'em for that little bit extra for a song. I personally haven't used the subharmonic feature, but these things are built like tanks. It's been outdoors, it's gotten caught in rain, and it's been bounced around car trunks. Can't ask for much more out of an inexpensive head.
The sounds I go for in the bands I play with are very Muse inspired, therefore I run two of these heads at the same time, each going to its own cab. One head takes my clean signal and is always on, the other is turned on and off with a very driven sound from my EHX Bass Big Muff. I take a DI from the clean head and mic the distortion cab, usually with an SM57. They have a very aggressive mid-range, making them perfect for rock and love to cut through, I enjoy their unique sound for what I do and I am luckily located a very short drive from Ashdown HQ in the UK.