The Plinius amp uses the RC5 IR protocol, device 16. Amps that use RC5 typically use 16, and furthermore most units use the same function numbers for Vol+/- as does Plinius. However, normal practice is to send 13 for Mute, and Plinius uses 54. I've verified that the function numbers you've posted are correct.

Many universal remotes will control Vol+/-. Most universals (including the VL600) can't be made to send 54 for mute. However, remotes made by UEI can be made to work.
Nearly any OneForAll remote, some Radio Shack, Comcast, Atlas remotes used by other cable companies, high end Vizio, and some RCAs will work.
If you want to buy a new remote, the RCA RCRP05B ($20 at Best Buy and Walgreens) is a good choice.

For the RCRP05B, use setup code Audio 1189
Vol + and - should work immediately. To make Mute work:
Tap Audio
1) Press Setup until 2 blinks
2) Enter 994 (2 blinks)
3) Tap Setup
4) Enter 29409
5) Tap Mute
The remote should blink twice.

If you have a different remote, the required setup code, and 5 digit function code may be different.