I've had a couple of stock Dynaco ST70's, and there have been some mods or even redesigns which addressed the flaws in that very historically-important and influential amp. Some of Bill Johnson's early ARC designs were variations on those of David Hafler (particularly the SP-3 pre amp)---even at the time of the SP-3/D75/D51, Johnson was selling modded ST70's, and Frank Van Alstine at one time offered an extensive mod which eliminated the ST70's major flaws.
In contrast, Roger Modjeski started with a blank slate for the RM-10, creating a completely new design, not an updated version of a design from the 1950's. Why anyone would want an ST70---old or new, stock or modified---rather than an RM-10 is a mystery to me, especially when an RM-10 can be had for the same price as an ST70. What if a Porsche cost the same as a VW? ;-)
To view the historical significance and number of Dynaco ST70's sold as a reason to consider it worthy of consideration today is akin to doing the same for the Acoustic Research 3a in the field of loudspeakers. Good for it's time, but far surpassed today. I regularly see pairs of AR 3a'a offered on Audiogon at a price that can also buy one a pair of Vandersteen 2c's. Which loudspeaker would you rather own?
