Why? simply that it adds bass I did not think this amp had in it. The bass extension is not subtle. The sound fills out, bass deepens to levels I have heard in other amps but not at the cost of being bloated. The bass we have come to know and love about this amp, textured and detailed still remains but it just goes oh so much deeper, considerably deeper than my last benchmark, the Mullards.
In combination with the Berlin made diamond base Telefunken DF96's which also add bass quality this is just about the most perfect combination to these ears I have heard with this amp. All the pace rhythm and timing is there, the snap and immediacy, the delicate touch in the mids, the sweet crystaline treble but now we have the bass come in to play.
Now, the downside to these tubes is they are made of the same unobtanium as the Marconi's but honestly, if you ever see a pair, grab em! Simply, with all the 3S4 tubes there is nothing to compare.
The added bonus is these are inky black silent tubes. Not a murmur, not a rustle, no ting tings just inky black. Oh, and another thing, more defined soundstage back to front as well as separation of instruments within that soundstage.
I can see why Shikada San has raved about these tubes for so long. Now, all we need is the industrious Japanese members of this forum to go hunting for us. Somewhere in Japan in a dusty wear house must be a box of these waiting for new homes!