Amperex Bugle Boy E88CC
- Adds treble definition without raising overall levels or changing the timbre much.
- Very good detail compared to stock.
- Can potentially be fatiguing, but I did not find this to be the case with most recordings (and I'm treble-sensitive).
- Mids and bass essentially unchanged from stock.
Telefunken E88CC
- Incredibly smooth. If the stock tube is butter, this is just velvet and liquid at the same time.
- Not as laid-back as stock. More forward, but interestingly has a deeper stage that might be slightly wider.
- Adds some bass slam, but doesn't increase levels.
- Is more colored than stock, but in a way that is pleasing and musical. Best heard with passages with guitar reverb and good classical orchestral recordings (strings in general)
I tried 5 different tubes, but I felt only these two represented a substantial difference/improvement in the sound. It's worth noting that the Telefunkens are pretty expensive at this time (a matched pair costs as much as the amp), and thus I can't necessarily recommend them.
Honestly, in my setup the stock tubes are used 99% of the time.