Siemens EL 34

I used two quads in my Music Reference RM9 MK2 for about two years. I’ve since sold the amp, but held onto a quad for possible use in future tube amps. I consider them to be on the warm side of neutral... not thin, nor extremely warm. Bass is tight, but not as prodigious as Gold Lion KT88’s. I like them a lot... and they are dependable.   Good tubes for sure!

I'm not sure what you are goals are... but other considerations based upon my experience using Music Reference RM9 MK2 as the host amp:

• RAM Labs Chinese EL34's were very enjoyable, especially for the bargain price.  Very lush, warm and detailed... but mine didn't hold up as well as Siemens and Gold Lions.  However, if you don't mind replacing them more regularly (given the low price), they really did sound excellent in my experience
• Gold Lion KT88's (assuming your amp accepts them) sounded great in my Music Reference RM9 MK2.  Excellent bass, well balanced, detailed, with a somewhat darker sound.  However, I preferred Shuguang Penta KT88's (no longer made) in the VAC Phil 200 and McIntosh MC 275 MKV.
• Gold Lion KT77's are typically "drop in" compatible with EL34's were on the lean side of neutral... a bit more sizzle on top than all of the other tubes referenced.  Think clean sounding, not tubey.

My new amps (only 2 days old and still breaking in) are Quicksilver Mid Mono's.  At the recommendation of QS owner, Mike Sanders, I bought them with NOS Tesla EL34's which are full bodied with surprisingly strong bass for an EL34 tube with a darker sound signature.  Really still early to judge these tubes and the amps, as I only have about 15 hours on them and they are still breaking in.

Ultimately, system matching and synergy with your chosen amp will play a big role as to what you prefer.

NOS Mullard EL34's are supposed to be the ultimate, but they are very difficult to find and very expensive.  As an aside, Mullard EL34 re-issues where the worst tubes I ever purchased... clear as mud, soft... just awful sounding.

I was not familiar with Tesla EL34's until Mike Sanders @ Quicksilver recommended them with his Mid Mono's... but I'm not sure if he's selling them to the general public, but they are listed on the Quicksilver website.

Thanks for the lead... I've been doing this a long time and I had not previously heard of HiFiTown.

Do you consider your Amperex/Mullard EL34’s to be head and shoulders superior sounding to your other EL34’s and 6L6’s?

I'd say you are pretty well set on tubes... no need to try more unless you are a collector.

I recently sold my RM9 MK2, despite many telling me to never sell it.  Next to a VAC 70/70 Signature (eight 300B's in push/pull), it was the best amp I've owned.  Time will tell if I regret it.  I have a pair of Quicksilver KT Mono's with KT-150's on order... so I'm hoping they will scratch my itch and quell any regrets I may have in selling the RM9 MK2.

@decooney  another benefit to the QS is I'll only be swapping out four power tubes vs. eight power tubes in the RM9 MK2.

Good to know I should expect a 200-300 hour break-in before the KT150's are settled in.  

@decooney I tried Gold Lion KT77's back when I owned a Music Reference RM9 MK2.  I preferred Gold Lion KT88's and Siemens EL34's in this amp, as the GL KT77's lacked emotion and just sounded too clean & lean in my experience.  YMMV.

@jjss49 both the KT77 and KT88's that I have owned are the current re-issues, which I've heard pale in comparison to the originals.