Good 61NP replacement tube for NovaPre?

My Peachtree NovaPre (pre amp not integrated) will be returning this week from Signal Path from warranty repair. I though while I have it out of shelf, maybe a new tube would be fun to try. To be honest, I cannot tell much of a difference between tube and SS when switching (remote has tube off button). According to specs, the unit ships with a 61NP tube. So with that info, I am always looking looking to warm up and extend low end if possible (I know it may not make a huge difference). My current system NovaPre, B&K ST140 (last version), ZU Omen speakers (latest version), Oppo 103, Apple TV, and Canare 11 gage speaker wire.

Don't get me wrong, I love the way my system sounds. Just getting the urge to tinker. Also I wrote upscale audio...but no reply (afetr a couple of weeks).