Reverb is always worth a scan. I got my Peavey Alphabass off there - 160w via 6 x 6L6GC, lovely. Theres always cheap Sound Citys on ebay and the like for around £350-£450 but not great for bass unless its the Mk3 models (in which case the price goes up). Fender Bassman 100/135's often pop up and with only a 4 x 6L6GC power section a full revalve wont be TOO much. If going vintage I would always set aside say £100 for a tech visit, plus the cost of a revalve just in case. Id also check out the very cheap Bugera 1960, whilst its marketed as a guitar amp it worked fine for me on bass, 100w via 4 x EL34's