I'm a WA5 -LE owner with a few mods. I've been listening with low impedence GS-1000s, medium impedence Senn HD-650s, and most recently, high impedance AKG-K1000s (all are Stefan Art Equinox recables).
All phones sound outstanding on the Woo, but to my ears, the AKG-K1000 using the high impedance jack is my clear favourite and the Woo sounds heavenly with this combo. It's taken me some time to get hold of the AKG's, but I'm so glad that I did as I believe it's a very special combination with the Woo 5. With many recordings the GS-1000s sound really, really good with this amp and on well-balanced discs with no leaning towards brightness the Grados are immensely engaging and have an enormous soundstage with prodigious levels of detail and low-down grunt. The SAA Equinox recable really helped bring the mids forward to create a sound which feels perfectly in balance. It gets you searching through all your recordings and leaves you with a big grin! My problem with the GS-1000s is with around 20% of my recordings which may lean towards slight brightness. With this material I have to take the GS-1000s off my head as the HF spikes are painful to me. That really tempers my enjoyment of the Grados with the WA5-LE and as a result I've not been listening to the headphone rig nearly as much as I should (criminal really). NB My GS-1000s are probably 500 hours burned in so I think they have reached a realtively steady state and should hopefully have completed the burn-in process fully.
The AKG's plugged in to the high impedance pot have solved all those problems. I've only had the combination for 2 days but my optimism for the Woo is now 100% back on. I can now listen to all my albums again. I have not heard the AKG's without the Equinox re-cable (WA5-LE did not have the XLR connection for the unmodded AKGs), but my initial impressions with the K1000and partly burned in cable (30 hours into an 80-90 hour process) is that I now do not need to perform any more upgrades on my system. I can happily imagine being content with this combination for many years to come. The low-end has much more impact than I imagined it might have, given the open design and distance from the eardrums. My favourite part of the K1000's sonics is the mid-range. Vocals are beautifully etched out and just cut through the overall presentation with perfect emphasis. The sound is just so balanced. The K1000s may miss a tiny amount of detail retreival and soundstage compared to the GS-1000s but I prefer the AKGs any day of the week. Why? Because they are just so perfectly balanced and have made me want to audition every single recording again and again. Two days since I've had the phones and two days of very late night listening sessions and more than a little red wine
So, ultimately my experience is of course representative of both the WA5-LE and the headphone sonics, but I would say that I did not know just how good an amp the WA5-LE was until the K1000s arrived. I've now reached the end of the upgrade path, and I'm very happy for it.