I just faced a similar problem, except my budget was 300. I searched on Ebay for months, tried out the Vox Valvetronix, and finally settled on a marshall avt or valvestate II. I ended up buying a vs265, which is a valvestate II with stereo 65w, which I really like. Comes with a preamp tube, and if you put in some good celestion, it almost sounds like a real tube amp.

Those avts, off of ebay you could get a 50w one for about 250.