Feel free to ask anything, anytime... I'll do my best to give you an answer that falls on the side of caution all the time.
Your Chi-nee 3A'er will most likely power the 2.5 perfectly fine, just remember that tubes can possibly draw up to 3x the rated current at startup. I believe those regulators have an over current protection built in so at the worst, it would just shut down.
I always find that running things close to maximum is never any good, heat is the major killer of electronics, so you might have to put a heatsink on the regulator if it gets too hot.
Just like the a 4 cylinder engine, it will last a lot longer running it at 3000 RPM than at 6500...
I would order an 8A or higher off of eBay and never worry about the current draw.
Unless you break the solder on the terminals with over enthusiasm.
Try the 3A first and see how it fares.