There are a lot of wonderful tube headphone amps out there and I've had several - Woo certainly makes some great ones in varying price ranges, and I've had that Quicksilver HP amp that @johnss mentioned; great bang for the buck, that is! But there are also companies like Ampsandsound, Decware, Donald North Audio in the US, and Auris Audio and Feliks in Eastern Europe who make great gear worth investigating (though Decware and DNA have long waiting lists). WE 300B tubes are also excellent.... I even drive my Susvaras and Modhouse Tungstens through some of these relatively low wattage amps and they sound just fine.
Where to start upgrading?
I've got the itch to start a gradual process to upgrade my setup, but analysis paralysis is setting in...
The receiver and speakers are less important to me as 90% of my listening is through headphones in a finished attic space with less than ideal acoustics. My gut reaction was to upgrade the turntable and cartridge first and build around that "base". I've been looking in the price range of ~$5-6,000 for the turntable and cartridge, but would a turntable upgrade have a low ceiling for performance improvement based on the other components?
My current setup is:
Turntable: Pioneer PL-560
Cart: Denon DL-110
Phono stage: iFi Zen phono 3
Headphone amp: Schitt Magni
Headphones: Sennheiser HD650