given a 2A/6.3V winding, there are a bunch of nice choices. the beefy EZ81/6CA4 is going to require about an amp. The octal 6BY5 is going to take about an amp and six-tenths. 6X4 and 6X5 are going to take less, and also contribute a bit more voltage drop. A 6AX5 will fit, but a 6AX6 won't. The WE412 is another neat little 9-pin, and the 6754 Bendix Hy-G-300 will also give a ~45 second warm up for the rest of the circuit due to its wonderful cermanic heater sleeve. The Bendix is also graced with a heater-cathode voltage rating exceeding its plate rating.I would go with the 6CA4 as a first choice. 6AX5 a close second, tied with the 6BY5, also an octal.
regards,
Douglas