FX Audio TUBE-03 Preamp Buffer

I believe they both use the same PCB. Open the case and check if the opamps are socketed in or soldered. If it uses a pair of NE5532 opamps, be sure to get compatible dual DIP-8 opamps such as the MUSES8920, LM49860, LM4562, etc. which are all good opamps. Next level opamps are the discrete Burson V5i-D and the Sparkos SS3602. Don't get the ones that are too tall or too wide.

Can this be done with the fx audio tube 03? I want to upgrade my op amps too

I upgraded the same preamp (same internal components though with a different brand name) with a pair of Burson V5i-D opamps (these fit inside the casing) and a pair of Vokshod 6J1P-EV military tubes and replaced the power supply with an iFiPower linear power supply with excellent results. While "audiophile purists" would frown in this, I like that its tone and bass controls give me the flexibility to compensate for poor recordings and a less-than-ideal listening area.