Thoughts on replacing caps in 30 year old speakers

Stolen makes four basic grades of cap, in ascending order: metallized polypropylene (black), tin foil and polypropylene film (white), metallized teflon (blue), and tin foil and teflon film (green). They sound very different.

Metallized poly is the least expensive of the audiophile grades. It sounds relatively smeared. Tin film and foil is a big improvement in clarity, but this may not suit your system as a whole. I find that teflon adds a slight edge, which may make your system unlistenably sharp, or much more life-like, or something in between. As ever, YMMD.

In critical applications I divide the critical value into 3: 1/3 Solen teflon f&f, 2/3 styrene. This gives me a slight edge from the teflon, and a slight rounding from the styrene, for a pleasingly neutral sound. Unfortunately Solen does not sell styrene.

Not only would I do it, I have done it. As you can infer.

To clarify: I have upgraded KEF's, Magnepans and Quad ESL's, not Thiels. But if you like their sound, why not? 

When I upgrade to film and foil, I expect increased clarity and decreased harshness. I have found that polypropylene is rounded, polystyrene is very slightly rounded, teflon is slightly edgy. I would never replace old metallized with new metallized. I get good results with MIT Multicap f&f. YMMD