Ok I've never heard Melz. Nep has the Melz and for some reasons he has forgotten about it and bought 2 pairs of ECC31. That is very telling.
My best 6SN7s are probably the Sylvania Gold 6SN7WGT and also the 7N7 Raytheon. 7N7 are Sylvania made...the early Sylvanias and they sound sweet.
However none of these are close to the ECC31 according to my ears.
ECC31 ...my impressions. You get a wall of sound coming at you and you feel closer to the music. Some says vocals are a bit recessed which I cannot understand why. I think that the mid range and vocals are where it shines. However that's not fair to the sub bass it produce. It has subterranean reach and impact. I don't like sharp trebles but I find the ECC31 high notes smooth and musical.
Also on my HE560, I prefer ECC31/5998. I think Nep does as well with his HD800. It provides the lift in bass and across the FR spectrum. The HE560 now has the oomph.
I believe the Melz are good otherwise Feliks Audio wouldn't be offering them as upgrade options. I would like to try a pair at some point.