Each tube increases chance for noise
With paralleled output tubes, this hardly matters. The SNR spec 90dB / 100dB is excellent. You won’t hear any noise unless you grossly mismatch this amp to an IEM. It would be a different story if this was a preamp using all the tubes to stack gain a mile high (most likely to be thrown away by the volume control).
Something like an ES Lab ES-R10 would be a good real-world test of noise floor audibility for this unit - 32 ohms, extremely sensitive (for a full size headphone), and a great sounding headphone with a price matching the amp. I expect it would be dead quiet.