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The MKIV will continue its ups and downs for 300-350 hours. And you really won't hear all it has to offer until around 600. What you are beginning to get makes sense... more refined bass and the tube magic... lucidity and natural harmonics. By now your tubes are probably about burned in...Some of those power tubes can really take a long time...and the circuits, connections and transformers. It all takes time. The refinement will continue. And Bass is the last to pull together fully. I would be really surprised if the Torri bass were not enough once burned in and tubed to your tastes, but it is somewhat quirky with some speakers. If this worries you, call Steve and he will walk you through it.
Also, don't think of the switches as anything but tuning devices. Everything will be fine no matter where you set things. If the back impedance setting is better, use it. If the bass switch off is better, use it that way. These preferences may change after another hundred hours too. The amp will clarify across the spectrum and quit those extra warm and extra lean segues.
Rectifiers are very good tuning tools and they all sound different. What is good? I find the Phillips 5R4s powerful and extended, but finally I miss some inner delicacy. To change Recs, or any other tube, begins where you are. And unless you know the characteristics of the tube, how it compares to what you have and where you would like the sound to go, it is a shot in the dark. May get lucky, but you may not like them. I can see how the Phillips 5Rs could be good with Siemens EL34s, since these 34s have rich mids, textured by a pretty pronounced upper mid bump in the MKIV. So the solid power of the Philips might work fine tuned up by the rich textured EL34s. But if the EL34 is not your thing, say you like tighter bass, or your system/room tends to brightness, or if you are like me, preferring the powerful, relatively linear, open lucidity of Genalex KT66, well that is different.
In this context...though I would replace the Rubys, I personally would not use a 5Y3GT in the Torii. The Tungsols and RCAs are beautifully tones tubes, but they flatten the extraordinary dynamics of the amp while also reducing bass. But if this is what you need for your system and tastes, then they could be just the right choice.
When the sound is this refined, everything matters and everything effects everything else.
I agree the Rubys will need to go, but I wouldn't want to recommend a tube without knowing how you would like your sound to change. And you really won't know your sound for a while anyway!
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