Stax SRM1MK2 I also use does have to be opened to the 3 o'clock - if not to the very max - with most combinations of phono cartridges and phono preamps. If that is not enough, you can try inserting a line stage preamp between the phono stage and SRM1(MK2). SRM1MK2 does like to be powered by a very low output impedance devices, otherwise its dynamic potential is not fully realized - despite being on paper a non problematic high impedance load. Phono preamps generally do not provide
low output impedance.
ART DJPREII offers max 45 dB gain and that goes up to about what you described. I would not bother regarding the position of the volume control being that high - unless experiencing too low sound pressure level with below average modulated records and/or overall unsatisfactory sound.
While Nagaoka 555 is a decent cart, it is *perhaps* not up to the resolution Stax is capable of. As I decipher you are on a rather tight budget, I would wait until sufficient finances are available for a better cart ( > arm?>table?>...) - volume control position would be the last concern . Besides that, phono preamps that offer gain adjustment in small steps (3 dB or there about) AND freedom from overload AND above normal high output tend to be costly, as they require higher voltage(s) to operate from . The moment one steps outside/above the realm of (approx) + - 15 V DC supplies and op amps prices start to skyrocket. Tube phono preamps, although by default working with high enough voltage, generally offer high output impedance, something that does not yield good results with SRM1 (MK2).
Try as much as you can BEFORE buying - you may well discover that improving upon present level does burn a substantial hole in the pocket.