Installed the "pristine" Amperex Holland "Bugle Boy" tan base NOS 1961 GZ34/5AR4 rectifier tube (from Andy Bowman at Vintage Tube Services) in the power supply of my Modwright SWL 9.0 Signature, along with the Bendix 6900s.
I ran them for 8 days or so, listening to various types of music on SACD, DVD-A and CD, university classical FM station, and cassette tapes.
The Amperex also brings more clean, taut bass content than the Sovtek, but not quite as strong as the cryoed Philips metal base NOS 1957.
Again, like the Philips, the Amperex adds a bit of midrange bloom but not bloat.
The Amperex similarly adds some relaxation and sweetness
to the high frequency energy of the 6900s, but perhpas a hair less than did the Philips.
The Amperex brings much the same character to the music as the cryoed Philips metal base,
but does not seem not quite as strong in the bass to my ears.
The Amperex brings significant improvement in performance over the Sovtek.
I have returned to running the cryoed Philips metal base NOS 1957 and the noticeable difference after the Amperex was the somewhat stronger low frequency content (which is indeed a bit better match with the Bendix 6900s in my opinion). The resolution and smoothness seem very similar between these two.
The Amperex Bugle Boy tan base NOS 1961 will now serve as a worthy "backup" for the cryoed Philips metal base.
However, I feel its effect on the music and its significantly lower price (compared to the Philips metal base) would make it a wonderful contender for primary use over the stock Sovtek.
I have not had any Mullard 5AR4 to compare.