Well, I've not heard the SLI-80, but I own, and love, the Sixpacs. The bass is very good, IMO. Not as much impact as a 200 watt SS, but much better than a lot of tube amps, again IMO.
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The SLi-80 has 80 watts of power in ultralinear mode and 50 watts in triode. The Sixpacs are 50 watt triode only amps. I own the Sli-80 and love it, but haven't heard the sixpacs so I can't compare the two. You're not going to get the bass of high powered SS with either amp - or most tube amps for that matter. Obviously, the Sixpac's sound would also be influenced by whatever preamp you select, so a direct comparison would be somewhat subjective. Another point to consider is that the SLI-80 is quite flexible since it has sub outs, a highly regarded headphone section, you can switch between ultralinear and triode on the fly and the amp will accept a wider range for output tube types (kt88, 6550, el34, etc.). If you research further here and over at AA I think you will find many fans of both amps with few detractors, so you can't go wrong with either, IMHO.
Get a CaryV12R. I replaced a McIntosh 7270 270 wpc power amp with one and I only need the 50 watts in triode mode. I don't really use the 100 watt ultra linear mode. As far as distortion forgiveness, 50 to 100 watts in a tube amp is about equal to 150 to 300 watts SS. Hook one up to a Cary SLP98P preamp and you can't go wrong. polk432
I own an SLI-80 Signature and have nothing but praise for it. As Cruz123 points out, the flexibility of this amp is impressive. Headphones. Sub out. On the fly shifting. The output and signal path tubes are fun to roll and experimentation really pays off. I keep a few sets of KT88, 6550 and EL34 on hand for different moods. The amp is simple to bias, and runs quiet. It's well built and reliable.
For more umph, however, you might want to find a SLAm 100; they appear here occasionally.