A question about my Rogue Sphinx II amp

Hey all,

Just starting out building a new hi-fi system. I picked up a Rogue Sphinx II second hand after demoing it at a hi-fi store. The amp sounded amazing compared to others so i found a second-hand deal as many of these forms suggested buying second hand as a first system.

Anyway, so I don't have my speakers yet.. so I've been listening to records through the headphone jack on the amp. 

One thing i noticed was after listing for about 10-15 minutes, i feel like i keep needing to bump up the volume on the amp. I know the whole ear fatigue thing, but I'm not really playing the music that loud. I'd start out at about 30% volume, then 10 mins later need to go up to 50%, then eventually im almost at max volume. 

Is this normal? Do i need to change the tubes?