I've run a lot of tubes through this amplifier. However, my impressions may be of limited value because I have a bi-amped system. I use the Line Magnetic 518IA to power a pair of Voxativ AC1.6 with two Hypex modules taking care of four 15" Neo woofers. A derivative of the Pure Audio Project speakers except I use an active crossover and I bi-amp mine. The speakers cross at 200 cycles. If I mention a tube, I listened to it in the LM518IA. Comments are relative to this amplifier, not the tubes in other applications. Still, hard fought biases are hard to overcome and I find myself more optimistic about the sound of NOS tubes. Crazy. I had these tubes in my collection and it helped me pull the trigger on this amp. Great for tube rolling for sure.
I'll give a crack at describing my experience. The stock tubes are very good. This is a great amplifier. No gripes with the sound but I like rolling tubes.
- I use a NOS Phillips metal base 5AR4 but I also used a NOS Mullard. Get one of these and be done with it. Then don't part with those tubes until they fail.
- NOS Mullard 12ax7 long plate f91 are staying in the amplifier. I can think of no reason to remove them. I also tried some NOS Telefunken smooth plates and didn't care for the sound. Also tried some NOS Sylvania 12ax7. They were nice but not as nice as the Mullard. Stock tubes are pretty good though. I'd play the stock before I'd play the Telefunken - in this amplifier.
- Overall, I prefer two EL34's the best. Remember that my Line Magnetic is not responsible low bass because I'm bi-amped. Best of all are NOS Mullard xf2 followed by the SED wingC (really a nice tube). These two provide an open midrange with texture and punch. Nicely extended top end. These worked the best for my application. Both of these tubes are much better than the NOS Siemens EL34 I have.
- New production Gold Lion KT77 these tubes got my attention at first. The most extended, clear highs of any tube I rolled. I could live with these for sure. I found this tube very detailed and dynamic. However almost too dynamic. An excitable boy. Gives up the midrange smoothness of the EL34.
- After the those, I liked the NOS GE 6L6GC. I will likely roll these in from time to time. Super transparent tube. The highs sometimes give that chimey shimmer that I love. Nice foundation without being too thick in the mid-bass.
- Soviet 6n3cE is an interesting tube. I like them. They are neither as bad or as good as most of the opinions I read. Pretty good value in my opinion. Closer to the 6L6GC.
- Overall, I found the larger tubes (KT66, KT88, 6550) to be too heavy sounding. The bass/mid-bass became too weighted and thick. Bass lost some definition. Just not as nimble sounding and fleshed out as the EL34. The TAD KT66 is a nice tube that is really clear and dynamic. I could definitely live with these but I'd have to tweak something else to tame the boom. What I mean is that the KT66, KT88 types sound bigger and very much more forward than the EL34. The TAD KT66 are very forward sounding. I likely won't spring for NOS KT66 based on my experiences so far.
- To me the Gold Lion reissue KT88 was ok with a big presentation but this would be my last choice. After the stock.
- Svetlana 6550B. These were from an Audio Research amplifier I owned in the 90's. No bueno.
I listened to all of these with the stock 845. They sound fine but I'm leaning toward buying the Psvane 845WE. Elrog are expensive and seem like a nightmare to own. I can't go NOS on these. $300 bucks for a tube is one thing but $1500 is another.