Siemens EL 34

Try checking with fellow audiogoners pdreher and clio09.  clio09 is now RAM Labs and a great guy very knowlegable.

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.

Thanks for your insights.  My amplifier can use 6L6 or EL34 tubes.  I find myself going up and back as each brings different characteristics to the sound.  Interestingly I am currently running 5881; which to my ear sound the best.  But I have noticed the NOS EL 34 world is drying up and wanted to get one more quad for the future.  The RAM EL34 NOS and Tesla both seem interesting.  Do you know if anyone is still stocking the Tesla tubes?