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I get what your saying Tom in regards to the tone being better. I liked the OG 5150 & 6550s I've had for metal, but the quality of the tone is better on the SLO (100w or 30w). It's like Wizards, there is just a certain quality, natural/organic quality to the sound/tone/gain structure/ Many more examples, but I like using Wizard as an example because it seems to agitate some folks that Wizards get talked about so much.
As far as available gain, I don't know about the OG SLOs, but the BAD 100w I had and SLO30 I have now have stupid amounts of gain/saturation. I play the 30 with gain at 8-9 o'clock & lightly boosted and it's still plenty.