i use a modified 502b that i bought from an agon dealer. easy to bias, other than unplugging outputs to components it is a 2 minute job at most. new ones have built in biasing option i believe plus remote. has worked great for me and has 60 wpc which is plenty. fwiw, if you are serious about sound you better trash those speakers before you can appreciate any decent amp, regardless of tube or solid state. i hope you are already aware of this :)