I recently noticed the discussions in this forum concerning early 5R4GYS failure in VTA amps. This spurred me to verify that I could at least salvage the tubes for use in the SET-300B. The spirit, as well as some of the personnel, from Sonic Frontiers days lives on at Parts Connexion, so I emailed the site for that company and inquired. Chris Johnson and another staff member named Glenn informed me that the 5R4GYS will suffer early failure in the SET-300B because "...input capacitance load is too high for this tube - it will burn out prematurely."
So we have a reason for early 5R4GYS failure for an amp not made by VTA. I do not have the electrical engineering knowledge to determine that the problem identified by the Parts Connexion staff also causes early failure in VTA amps. In my opinion the limited warning in the Upscale Audio website, i.e., to not use this tube in Bob Latino's amps, is at best incomplete. I think Upscale Audio should advise prospective 5R4GYS buyers to ask the manufacturer of the prospective buyer's amp about compatibility. Perhaps Upscale Audio should also research the matter further, so that more detail could be provided on the Upscale website.