You can use a preamp before a headphone amp, but it’s rare. Usually headphone amps include a volume control and have plenty of gain on their own, so it’s not a common pattern to stick in a preamp - with needless gain and an extra volume control - ahead of that. In particular, high quality volume controls that don’t degrade the sound are expensive - so why stack 2 of them? And if you have a DAC as your only source (as is the case with many headphone users), often that makes 3 ways to control volume.
I think ECP Audio made some headphone amps to order with an option for no volume control. I’m struggling to think of other examples. For electrostatic headphones (a niche within in a niche), some have used preamps to "flavor" the sound (i.e. a tube preamp to add thickening low-order harmonics), and the extra gain can come in handy here, since electrostatic drivers need unholy amounts of voltage gain. Also, "pure" tube electrostatic amplifiers are EXTREMELY rare - most of the e-stat amps with tubes are hybrids. The preamp can be used as a slot to add "moar tubes sound".
In some ways, a headphone amp is quite similar to a preamp. They both handle "line level" voltages and include a volume control. However, where they differ is that HA’s must drive much lower impedances than preamps. A preamp with 1K ohms output is generally OK, since it’s driving amplifiers that present a 20K+ load. That doesn’t work with headphones. The HA’s output stage needs special consideration to drive loads down to 32 ohms. Usually this means either SS output devices, very large coupling caps, or output transformers (OR, Berning's ZOTL). In early head-fi days the OTL (output transformer-less) tube amps were popular, becasue they were so simple. BUT they had very high output impedances, up to 100 ohms or more. And WOW could you ever hear this. It was a "euphonic" coloration, to be sure, but a strong one. They fell out of favor for this reason (also choosing appropriate output caps other than electrolytics was hard).
You can easily go the other way - use a headphone amp as a preamp - but the HA’s output stage is usually optimized to drive much lower impedances that it would ever encounter into a power amp. It will work fine, and often sounds good, but a line stage specifically optimized to be a preamp should usually sound better.
