Randall's yellow caps are excellent radial caps but
polypropylene, while I'm pretty sure the Mallory 150s
are polyester film caps.
They only sound very very "slightly different" to me
anyhow but both are very good in guitar amps,
depending on what you are building.
I like the 150 series in my tweedush amps but they
sound realy good in the BF Fender amps too!
A decent source of inexpenisve +600v polyester radial
caps is AES.
Bruce

For signal caps,
I agree with Kelley. I love Mallory 150's in the preamp.
Also, you may want to try Randall Aiken's caps. They
look exactly like the 150's. For this type of preamp, I
like non-polar .68uF second stage, non-polar 4.7uF first
stage cathode bypass. 1st stage coupling-.0022uF.
Tone-.047/.1uF and 250 pF Silver Mica. Have fun.
 
Jason C. Arthur

I don't hear
any real sonic difference between the two but I notice
less noise (not a massive difference) with the metal film
resistors. 
 
Trace
Voodoo Amps