I think you should post this question at www.vintagetubeaudio.com. They do restorations on that amp.
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I've got one too and love it. New it was about $1695. Price also depends on what tubes you put in there. I would say untubed or with garbage tubes in average condition it is worth $400 to $500. Good tubes help the value a lot. Also, original box and manual help.
Does the volume pot make any noise? Does the triode switch make any noise. How clean is the amp? How is the tube cage?
I may have an extra manual to offer your buyer if need be. I'm sure I could scan and email it for free if the buyer needs.
I will say my amp is mint with NOS tubes, box & manual and I would never take less $900.
I previously owned one of these amplifiers, and agree with Elevick's numbers.
Other than that, those here most likely already know, but it's basically an ST70, updated/upgraded to address the common complaints leveled against it. In the end, the question of whether it's actually a better product sometimes come down more to all agreeing it's certainly a different product.