So in the last few weeks I've been trying out some of the different resistor brands for the cathode resistor postition... and I'd say they've had unexpectedly large differences in sound.
Perhaps I should first list some other preferences and choices I have in the system. And also for starters I haven't really understood the statements that the mkVI has a bass weakness as I've never felt that way and now it even has my favourite bass of all my amplifiers for my
bassy planars.
My preferences for sound signature are that I like the Harman curve and I care most about bass, low treble/mids and high treble in this order and want them all to be good.
For the driver tubes I've been using the black and gold Shuguang 6SL7-T tubes nearly exclusively for almost 12 months. I've found these to have the best sub-bass extension, a very well balanced, detailed and smooth sound in all areas of the FR spectrum. The other tubes I've tried in addition to the Soviet stock tubes are the Tung-Sols in both the reissue and original forms and Full Music 6SL7 tubes and some apparently quite rare Siemens tubes that came with my second hand mkVI. My experience with the various tube brands here is not extensive so I'm naturally open for suggestions too but really like these 2012-released Shuguangs.
For the power tubes I have a quite wide selection and I've found my favourites in a combination of the Tung-Sol 6080s (in the "preferred construction") and white top Shuguang 6N5PJ. The reason why the Tung-Sols are good is that they have the best sub-bass of all tubes I tried. And the way in which the Shuguang 6NP5Js are unusual is that they have both the second strongest bass and also some of that big soundstage thing like the Svetlanas do. There are (at least?) three different variants of the Shuguang 6N5PJ: (1) One with clear top and metal base, (2) one with clear top and black base and (3) one with white top and black base.
Of these, the white top has some emphasis on midbass punch and low treble, the black base clear top has emphasis on sub bass and higher treble sharpness and the metal base is most neutral of them. The white top combines well with the Tung-Sols as the midbass and low treble foci add dynamics to the sub-bass emphasized Tung-Sol sound. In some combinations I've also used a combination of two variants in the black base clear top and black base white top, using the clear top variant as a kind of substitute for the Tung-Sols if they didn't work well in some configuration of rest of the amp. The metal base variant I use in my other OTL amp that runs two output tubes.
In the stock configuration of the mkVI, however, they did not sound very usable as despite their good bass they had an overly soft sound and they came alive only after installing the decoupling caps and film caps for the PSU output.
But back to the different resistor brands. I had had the Kiwame brand resistors for cathode and anode resistors for a year and was looking to try some other options for the cathode resistor so I got Ohmite Audio Gold ceramics, TKD metal films, Shinkoh tantalum, Audio Note tantalum non-magnetic and Audio Note Niobium non-magnetic to try out.
(For the anode resistors I have the Kiwames at 68k and most other resistors are either stock or Mills MRA.)
I had read over various forums and sources that the Kiwames have both very low tolerances (0,5%) and a "tube-like" warm, thick and bassy sound so that's probably what I had gotten used to. The first replacement I tried were the TKD metal films. I was quite surprised over their difference. The TKDs have a very crisp and clear sound that even sounds a bit artifical and they also have the lowest bass extension and quantity of these all. For my tastes I'd even say that they have a bass deficit. But they do have a certain character too that may be preferred for other people. Next I tried the Ohmite Audio Gold. It had just a bit more bass and less clarity and crispness than the TKDs. They ended up sounding a bit generic and bland. Probably not the favourite for many?
At this point I was wishing for a sound that would have the bass of the Kiwames but more clarity like the TKDs. I still had the AN tantalums and niobiums and Shinkoh tantalums. Of these I had read in advance that the Shinkoh would have been considered the best resistor at some point in the past and that they would have great mids but possibly lean-ish bass. And what I had read about the AN tantalums was that their sound was overly "tubey". The AN niobiums were the newest product of these, released only in the last year or so and there was not much information or impressions of those, but one that I remember had said they were the best of the three. So I tested the AN Niobium next and they were pretty much exactly what I had hoped for - almost exactly the kind of bass like the Kiwames except cleaner and also very good clarity and detail over the FR, a very refined, detailed and smooth high end kind of sound. Over a few days I began to notice though that they had slight recess in the mid to high treble - so I decided it might be a good time to do the input/output cable upgrade mod for the amp to see if it would fix that... and that's what it did. Afterwards I still tried the AN tantalums, but they were clearly worse than the niobiums with soft bass and muddied low treble.
So as they now worked in my system I'd rank them as follows: First, Audio Note Niobium non-magnetic - these are worth their price here and they are in a different league to the rest. Second, the Kiwames - they have good punchy and deep bass and are relatively cheap too even if they don't have the very best clarity. Third, the AN tantalums which had many things in common with the Kiwames but were worse. Fourth the TKD metal films which have a crisp character but are low on bass. Fifth and last the Ohmite Audio Golds which didn't really stand out with their characteristics.
The Shinkoh I still haven't tried but will probably do in the future. I might also re-test the Kiwames at some point as I haven't done so after replacing the wires. It would be interesting to try the Mills MRA here as well.
In any case these comparisons again show that there are many, many different possibilities with which to configure the amp to match various preferences.
(Edit: The tested resistors were in the 2W power rating except the Ohmite which was 5W.)