A quick call to Steve would very likely provide insights into any bias function and sound related to bias questions. I wonder if those of us without meters experience this without knowing? I would be surprised if 5AR4/GZ34 were not acceptable for the Rachel, but who knows, maybe you have a hot one. I ended up using JJ GZ34 in my SE34 (pre-Rachel) and really liking the GZ34/5AR4 dynamic character, definition and space. This was quite a few years back, but if I recall correctly, the JJs were pretty open and clear tubes for the type, fitting my preferences for open neutrality.
With my Toriis though (and maybe I would feel this way with a Rachel now?), GZ34s have been too much for my preferences and system/rooms. They work great for many, but for me they just push the other tubes a little too hard...not necessarily into distortion, but too much signal density, causing an overblown sound and reduced open spaciousness. The focussed density of instruments in space can easily fill the sense of space between instruments. There is a place where it is all "right," great tonal density with great ambient space, the right balance allowing greater fine detail also.
This is what led me to Phillip made GZ32s, having the GZ definition, space and speed, with less signal density than GZ34s...a toned down version of the type. But they are usually relatively expensive. Some 5V4Gs (American designation for GZ32) can be quite similar in balance and character, though having their own varied personalities too depending on make and vintage. Especially the earlier fat bottles with D getters can be really good tubes and inexpensive alternative to GZ32s. Like this:
http://www.ebay.com/itm/True-NOS-NIB-Sylvania-USA-5V4GA-Get-Black-Plate-Vacuum-T...The ones I have that look like this are a little less refined in terms of very fine detail, texture and complexity than Mullard/Phillips type GZ32s. But here they have quite a nice balance top to bottom, being more about openness and fine detail with a bit more liquidity than the fine detail with texture and openness of the GZ32...also a nice sound. My pair of these Sylvanias also goes a little deeper and is more dynamic and focussed than the Mazda labelled Phillips GZ32 I have been using, also nice in its own way. The focus and open clarity of the Sylvania can be a little too intense depending on other tubes and everything else, especially if your room/system tends to hot/saturated notes....like strong upper range piano hits or similar trumpet notes. Right now they sound pretty great here though...
6N1Ps are prone to earlier distortion in my Decware experience, and to a lesser degree, ECC88/6DJ8s can often distort easier than 6922/E88CC. I can't really remember how 62NPs fit into this, but it may be they are being pushed a little hard by the GZ34 bias change. Again, Steve could clarify this.
I ran some cryo'd reissue Mullard EL34s for quite a while in my MKIII. It has been a few years ago, but I remember they were in the "long burnin category" compared to many power tubes, and were particularly "not good" until they burned in more. I got mine from a forum member who gave up on them as unbalanced and unresolved with maybe 100 hours on them. When I put them in here, I heard the same burnin things I had heard before with slow burnin power tubes...and hoping this was the problem, kept burning them in.
In burnin, the highs tend to be harder, detail more consolidated/less complex, and the mids down, thickish, darkish, undefined, also lacking complexity. This makes an uncomfortable balance issue (for some tubes more than others) beyond lack of refinement....murkier low mids down contrasting too much with a sort of crudely defined/more articulate and open top. With these tubes, this imbalance was pretty bad I think...the upper ranges a little sizzley and the bottom sluggish and thick.
I think it was over 150 hours before I started to hear them. Then they got better for a while, the bottom more defined and clearer, and the detail more complex and natural...relative balance finally. Here, in time, I got to like them a fair bit at the time.
With each of our system/rooms so different, it is hard to guess the best synergistic combination of tubes, but hopefully these experiences will somehow help you find yours.