If I'm not mistaken the SL60 was actually built in Japan for Soldano, and is the Japanese version of the HR50 I had a chance to play one that was next to an SLO for comparison and it was really no contest - the SLO killed it. Same person had a Hot Rod 50 that I prefered over the SL60 even though they were supposed to be near identical amps. May have been tubes.
If combining amps for flavor is your goal my suggestion would be to look for something that will be different and compliment your Marshall. If you didn't already own the Marshall I would say its a safe bet. The SL60 is pretty Marshall-y sounding.
Your other option is to look into the Budda stuff, they are voiced much different than the Soldano's
In the end, try and get ahold of it and let your ears decide - it may work perfect for your needs. There aren't many available and if you get a good deal on it you won't lose any $$$ if/when you decide to sell it.
Steve E