Yes it is. The black stripe III has my favorite Lead channel of all time, and is the basis for 1/2 of my favorite Boogie of all time, the mighty Quad! :twisted:
I haven't checked this yet from the schematics but last time I looked into this I found it was a common misconception that a Quad is half a IIC+ and half a III Black Stripe, but it has some similarities between the two.
Regarding the III if you're keeping them stock similar to what lions posted before:
Black - Raw and aggressive. Original R2 channel with resistor R130 added only.
Purple - Less aggressive. More refined R2 with added cap and a gain boost resistor off middle pot. Personally my favourite and I think it's a very undervalued amp because people want either a Black because of the 105PT or the Red because it's 'closest to a C+'.
Red - Circuitry close to a IIC+. R2 refined more and now has treble shift enabled for R2 which causes some added noise when treble shift pulled.
Blue - Insane monster. Crazy gain. R2 same as before.
Green - Simul only. Pentode output. Like a less crazy blue. R2 same as before.
If you're modding them, then you can pretty much make any stripe into another stripe or a IIC+. The only other differences are 100/60W/Simul and the transformer types and date codes. To throw a spanner in the works I have a factory black/purple crossover which is a black stripe (black stripe component values) with a purple stripe board. Best III I've ever played hands down, but got even better when doing a III+ mod to it.
My personal view on the III is to make it what the C+ should have been - a factory C+ with a switchable ++ (apart from the 12 originally made). Do the R2 mod to balance the volume or do one of the switchable ++ mods where the R2 channel was before. They're very similar and the R2 channel evolved out of the original 12 switchable C+/++ models.
Other people have the view that the III is its own thing as of itself and that is also cool. Whatever you want - if it gets you the sound you want and gets your to play more often then that's the amp for you!