A Caution: 
The Amazon Chinese "ceramic" caps-
most are dipped blue or red- 
are complete junk- very lossy at RF.
Don't know about performance at audio. 
They must make the dielectric out of
floor sweepings or something.

I have found a couple of Amazon
electrolytics with the lead polarity
reversed, as has a friend, but I've also
installed them in equipment which has
been working reliably for years.
Just pay attention to the voltage rating. 
250 VDC does not mean 300.
Also have bought their resistor
assortments and had no complaints,
so far.

73 Dave S.