Hi Matt,
LTO6 is a hell of a data sink and needs powerful hardware to be driven to expected throughput (it got better rate matching than LTO5, though). Servers from the age of Win2003R2 may actually be on the brink of not being adequate, depending on the block size in use. For LTO6, I would recommend 512KiB or more, with 256KiB as the absolute minimum in case it is unavoidable for compatibility reasons. Everything smaller and a BMA may overload a CPU core before reaching a streaming rate, let alone LTO6 full blast. That's orthogonal to the need for multiplexing (or staging) as communicated by Bob, given it is hard to find sources that can conjur up data that fast, specifically from the age in question.
HTH,
Andre.