@roxy54 I'm not sure ARC prices have moved ahead by much more than inflation.
I recall the SP10 was $4,000 when launched around 1980. The Ref 6 is currently around $17,000.
$100 in 1980 is $360 now, so the Ref 6 ought to be $14,400, so not much less than its current $17,000 price.
Most people would say the Ref 6 is a better amp, but I can listen to my SP10 and there is much less difference than you might think. I have to listen on my second system as my main system is balanced from stylus to power output. The SP10 is definitely warmer, but then some who favour tube amps also favour 'warmth'.