I doubt you will easily find a 5.1 receiver that you would like as well as your PV5, especially given that you like tube equipment.
You could keep your PV5 and simply add a 5 channel 5.1 receiver or pre-amp, although it is a little inconvenient. A typical configuration is to have a 2 channel pre-amp that has home theater bypass inputs, which simply passes the L/R channel line level signals through to the amp, bypassing the volume control. That allows you to keep you 2 channel system in place and add a HT receiver. Since you do not have a HT bypass, you can emulate one. You can just put the HT L/R line level outputs into an unused input on the PV5 and then figure out the volume level on the PV5 that amounts to unity gain. Then each time you want to use the HT you just set the PV5 to that volume setting and run the 5.1 receiver as normal. That will allow to get a mid-level 5.1 receiver without compromising your 2 channel setup.
If you get a receiver you can use its amps to drive the backs and maybe use the receiver or your Threshold for the center.
If you want to simplify into just a 5.1 receiver or pre then finding one you like as much as the PV5 may be difficult. Marantz, Arcam, Anthem, Integra are certainly possible.
The real issue is how important it is 2 keep your 2 channel sound intact.
You could keep your PV5 and simply add a 5 channel 5.1 receiver or pre-amp, although it is a little inconvenient. A typical configuration is to have a 2 channel pre-amp that has home theater bypass inputs, which simply passes the L/R channel line level signals through to the amp, bypassing the volume control. That allows you to keep you 2 channel system in place and add a HT receiver. Since you do not have a HT bypass, you can emulate one. You can just put the HT L/R line level outputs into an unused input on the PV5 and then figure out the volume level on the PV5 that amounts to unity gain. Then each time you want to use the HT you just set the PV5 to that volume setting and run the 5.1 receiver as normal. That will allow to get a mid-level 5.1 receiver without compromising your 2 channel setup.
If you get a receiver you can use its amps to drive the backs and maybe use the receiver or your Threshold for the center.
If you want to simplify into just a 5.1 receiver or pre then finding one you like as much as the PV5 may be difficult. Marantz, Arcam, Anthem, Integra are certainly possible.
The real issue is how important it is 2 keep your 2 channel sound intact.