What kind of issues are you having with your 55? I have a 45 (1x12) and don't even want to think about having to replace it. It's a unique amp. I really like what it does. I can get it to sound like my Mark IV for certain tones. I can get the red channel to sound very "Marshally". It's a very versatile amplifier.
To find a replacement for it would depend a lot on what you use it for. What sort of music do you use it for?