At the moment there is limited stock at the well known suppliers but you dont need matched so should be able to pick one up. When you do find one you should hear a large difference across the board (imaging, dynamics, top end openness). In my set-up the stock tube is a large step backwards...
I dont mean to sidetrack, however, is the tube you are referring to a standard ARC supplied 6550 which is a new Tung Sol ? If so have you tried the NOS Tung Sol grey plate? I found that the NOS 6550 transformed the pre to another level altogether.
Every room needs acoustic treatment combinations (absorption/diffusion) and specific sizes and locations to improve the listening experience at the seated position(s) period. One can buy some low cost 2 by 2 panels and play around with locations of the early reflections and clearly hear the...