I use my Delos with 58 dB of gain with sufficient volume; in the past it worked fine with 54 dB as well. I find 63 dB makes the sound shrill and overall unpleasant so I would not recommend going over 60 dB on the pre, but...
I think the issue is with your speakers that have a rather low sensitivity, and possibly low or not flat impedance for the tubes to drive them well. When I switched from my 91 dB/8 ohm flat monitors, to 86 dB/6 ohm speakers, I also had to crank up the volume on my line tube preamp quite a bit to get sufficient volume even though I drove them with a pair of 180 WPC tube monoblocks. So in your case, perhaps more gain would work, but I'd recommend trying to fix the problem with a better match for your amps rather than more gain as 0.6 mV cart should not need more than 54-58 dB in most systems for best performance.
I think the issue is with your speakers that have a rather low sensitivity, and possibly low or not flat impedance for the tubes to drive them well. When I switched from my 91 dB/8 ohm flat monitors, to 86 dB/6 ohm speakers, I also had to crank up the volume on my line tube preamp quite a bit to get sufficient volume even though I drove them with a pair of 180 WPC tube monoblocks. So in your case, perhaps more gain would work, but I'd recommend trying to fix the problem with a better match for your amps rather than more gain as 0.6 mV cart should not need more than 54-58 dB in most systems for best performance.