Dear Orkney,
I've had a 300xs for a couple years.. The already nice stock tubes were replaced with EL-85 Mullards circa early 50's and same era Sylvania 5751 in mine. I have a set of 60's Telefunkens to try too. If I am correct, you need (2) matched sets, not a Quad. Jonathan Halpern was most helpful with this.
My speakers are DeVore Fidelity 3XL, using Auditorium 23 Speaker Cable. Really super great sound. This is fed by a Mac-Mini to Bel Canto 2.5 DAC and also a McIntosh SR-67 Tuner. I would like to tinker with other headphones, but have only used my Grado GS-1000i and Grado 80i...tomorrow I am going to try out a pair of Senni 595's. (I bought my whole dev team these headphones as a token of thanks for their hard work.) Even if the 595's aren't great, they will be an incredible step up from what they currently use. (now to find them an affordable head amp and DAC combo for the holidays)
To my ears, Leben is a fantastic sounding amp, headphone or other. Funny, outside of this forum most enthusiasts are buying Leben as an Integrated Amp vs. Headphone. The DeVore's are more musical and have more tonality than Harbeth, and I like the Harbeth British sound signature. I also own Sonus Faber Cremona Auditor M's. Very sweet, incredibly musical, but too difficult to drive to any SPL with the little Leben. Maybe the Leben 600, but I have never heard it. The Leben makes my Jazz and Blues sign, and I listen to lots of Alt Country that it drives the crap out of. Very engaging. Never fatiguing.
Did you get yours from a dealer? I believe I posted my concerns with the pennywise, pound foolish issue with Grey Market Goods. To each their own, and you won't regret your Leben. (if you ask the question I can't remember, but the XS version has upgraded internal components and cosmetics but I never compared the X vs. XS so I can't tell you the sonic difference.)
Cheers,
bret