When auditioning an unreconditioned "stock" St70, most people generally complain about the "muddy" bass and rolled off highs.
One must bear in mind that what he is hearing is NOT the ST70 sound but the sound of the 40 year old parts. Most of the resistors & caps are now out of tolerance due to age & heat.
All one has to to is to check the specs & replace these w/ new or NOS parts and whallah! you get one of the best sounding tube amps for the price.
Remember, there are basically 2 camps when it comes to st70's - those in the restoration camp and those in the modification camp.
Restoring means you replace the aged components with newer ones of basically the SAME SPECS. Some up the capacitance and voltage here & there, replace the RCA's, speaker posts and that's still considered restoration.
Some are into modifications: Uncle Ned's Triode Electronics board, Curcio, Welbourne, etc. Basically, all that's retained is the ST70's OPT's. Mods usually involve redesigned driver boards, power supplies, etc.
Most mod makers in the U.S. (esp. Joe Curcio with his 'premium board") claim -and their users swear by it too- that these modded amps can beat many high end, megabux tube amps (I won't name brands here). While some of those who belong to the classic restoration camp believe these mods do sound "better", it is not the classic ST70 sound anymore.
So take your pick. Restore & listen to the vintage ST70 sound; or modify and beat the hell out of your friend's mega bukol tube amp.
Cheers!