Rogue Audio Dark Upgrade.

Which amp, specifically? They do it on ST100, M180, Apollo, and even old Zeus.

I went from stock 2010 Apollos to 2017 Apollo Dark. To be quite honest, I feel the stock Apollo were a better value. If I could have this back, I would have stuck with the old Apollos. But everyone is different, and you will find others gushing about what the upgrade did for them.

Two caveats to my circumstances:

1. I had already rolled excellent NOS small tubes into my old Apollos. Which negates the biggest sonic upgrade from the "Dark" model (which usually comes with RCA clear tops and Telefunken smoot plates - there are better vintage tubes than this).
2. I don’t think I like the premium Mundorf EVO coupling caps that go into the Dark models (standard models use plain white M-Caps). I wish Rogue offered a couple choices of premium caps.

The best upgrade would be to move up to the next bigger amp model, culminating in the Apollo monos. It does make a difference over the M180, even if you don't need the power (I have 96dB Tannoys). Unfortunately, I think they only offer the Apollo in "Dark" form now. But Apollo and Zeus amps are a GREAT deal used, if you can deal with shipping / transport issues.

I am totally not recommending this (lol), but I’ve run a 12BH7 sub for 12AU7 in my Apollos for several years. I also have a friend that does the same in his Zeus. It’s a much better driver tube than 12AU7 in my experience. The driver tubes in an amp are often overlooked but can make a big difference. Sometimes it feels like 1 or 2 small tubes are struggling to drive all the KT’s (especially in Apollos). If you want detail & dynamics - this is the sub.

Mark from Rogue used to "OK" me that sub for a couple of components (Apollos and Ares - very hard to find quiet 12BH7 for the latter, though), but Nick has not been friendly to it lately. The issue is 0.6 Amp heater draw per 12BH7 vs. 0.3 A per 12AU7. In context of the 1.8 A per KT120, it’s not much extra total draw (is my reasoning).

Lots of good 12AX7 and 5751, and fun rolling them in the Rogue tube amps. Lots of different tonalities / balances that will work with different systems & ears. Roughly from brightest to warmest, here’s what I liked:

Mazda silver plate 12AX7
Telefunken Ribbed Plate 12AX7
Tung-Sol 12AX7
GE 5751 TM BP "silver clips"
Amperex 12AX7
Mullard 12AX7, CV4004

Despite being on the bright side, the Mazdas are actually my favorite. They have beautiful mids along with more detail and dynamics than the other tubes. It was like a full component upgrade to my non-Dark Apollos. With Apollo Darks and now slightly brighter speakers, it’s not the best match anymore - which kills me because I love that tube. I also recall really liking the GE 5751 "silver clips" version which was only produced for a couple years in the 50s (something like 52 - 54). Both Teles are decent, and easy to find. The Mullards end up being a little warm and thick for my liking. 

I’ve also briefly tried a GE 12AY7, that worked pretty well and lowered the gain (Apollos have a LOT of gain).

The original Apollos shipped with Electro Harmonix small tubes and Electro Harmonix KT90 - the result was a bright and lean sound! KT120, and any of the aforementioned small tubes, are much better.