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The sIII is a two channel version of the Marshall 18 watt which incorporates Treble, Midrange, Bass, Volume and Master Volume. The second channel is the Tremolo channel from the original design, without Tremolo. This amp is a medium gain amp and is voiced to do the plexi sound very nicely. The range of tone is from spanky clean to Marshall grind. You can get a great Clapton Bluesbreaker sound and the Hendrix as well. This amp can handle Blues-Rock, Country & Reggae. Think a little JTM45.

The 18 watt TMB is also a two channel version of the Marshall 18 watt which incorporates a Treble, Midrange, Bass, Volume and Master Volume. The second channel is the Normal channel from the original 18 watt design. This amp is a high gain amp that does the 80s sound very nicely. It is intended for musicians who want plenty of grind and distortion. It does ZZTop very nicely. It is both responsive and aggressive. This amp does classic rock crunch to metal 'chugga-chugga' . Think modern Marshall.

Now there is also the sIII Plexi which THUMPS and grinds with a little less compressed tone than the others. The normal Channel provides way more overdrive than most 18 watt normal channels. The TMB channel is capable of some of the best clean sound out of an 18 watter. Crank up the gain and you are in crunch tone heaven. And it has this THUMP that the other variants don't. It's that frequency that moves pant legs! The Plexi sIII is much more like the '69 plexi where you get great cleans low on the volume knob and full bore distortion as you get above 2 or 3.
Sounds very much like a great 50 watt plexi, it has way more girth than most other 18 watters. The clean boost adds another dimension and it nails all those old AC-DC tones dead on and has way more gain on tap if needed.

Since the v6 runs on EL84s and 6V6s dosent it have a sIII tone with the 84s and a "other tone" with the 6V6s?