What coupling cap will you tend to replace, is it the 0.05uF or 0.1uF? Mit RTX is a good choice but bit pricey.
Multicap RTX is still my favorite coupling cap for the Stereo 70. Smooth, grain free highs, sweet midrange and tight bass all rolled into one. High frequency extension is limited by the ST70's design though the mid is to die for. Bass is a bit flabby but bass lines are delivered with integrity. The 0.05uf can be replaced with a cap of lesser quality or the original be left as is as long as the value is still within the range. Next best is the polypropylene Auricaps. 90% of the film polysterene RTX's performance at less half the price. These caps are fast becoming the high-end industry standard.