There is no boost in the SL-X. It is dual master volumes, no boost involved.
I have a modded 100 watter SL-X and JCM 800 cabinet I'm trying to sell...
What is this mod you speak of. I might have to jump on it if its the right price.
I had the transformer replaced with a Marstran replica of a 1969 1959SLP transformer and the gain brought down a little for more headroom and power side distortion. Between both mods they beef up the tone quite a lot.
I'll sell my straight cabinet and the head for $1300 plus whatever it costs to ship. The tubes are fairly new SED "Winged Cs" (best new EL34 quite easily) and NOS German RFT ECC83s in the preamp which should last quite a while. I have a footpedal, not sure that it's original but it works.
And it does have the triode/pentode switch as well. Even with the mod it still won't quite do clean, but starts at mildly overdriven. I have not done this, but I would probably put a 12AT7 in the preamp to bring the gain down and give more headroom. I'm fairly sure the payoff would be large.
It's a good amp. I'm going to miss it, but I'm trying to upgrade to a Roccaforte DRRS.