Well it's now a 900 Euro cartridge, but that's not the issue. It's a big heavy cartridge that's probably not going to work on the Pro-Ject arm anyway. You need something with more adjustability that can accommodate an old school cartridge like the SPU. You usually see these cartridges mounted on...
The low damping factor and high output impedance of these budget single ended amps really aren't a great match for modern vented speakers. Bass will quite often sound bloated and will lack definition. And the low power means there's very little headroom.
jeff
Appears to be cathode biased, so there's a good chance the 6P3S-E's will be running way too hot in your amp.
I don't see anything on their website about running other output tubes.
jeff