Best way to add remote volume control to tube pre.

An external box is convenient, but if you want the best sound, take a close look at motorizing the internal stepped attenuator-- or perhaps even replacing the stock attenuator with a better attenuator in conjunction with a motor drive. Assuming you're handy, you could buy a great compact SMD attenuator like Goldpoint or DACT, together with a motor drive, and achieve better sound than you're getting now, at much less cost than a Placette or equivalent RVC, and without the degredation and cost of additional ICs.

If the pre has or can be refitted with a high-quality stepped attenuator, Bent Audio makes a nice motor-drive/infrared remote kit.