Klots said:
Mullard CV4003 (12AU7?) would be ok? ~80$ from UK
That GE 6AS7GA would be a bad choice?
The cv4003 is nice and often can be found for a lot less I regularly see them go for around $30 here in the UK.
The Chatham 6AS7G is a nice sounding tube for the money $19 below, I prefer it to my rca ones that's for sure.
http://www.ebay.co.uk/itm/Chatham-6AS7G-JAN-Black-Plates-Test-Strong-/251488832521?pt=Vintage_Electronics_R2&hash=item3a8de70c09
http://www.vacuumtubes.net/ have a couple of hundred nos 5998's at $85 each while stocks last
You should be able to find a nice 1950's Tung-sol 12au7 jtl for around $15 one of the best sounding input tubes I have used in my Crack its very similar sounding to the Tung-sol 12u7 Black glass which goes for a lot more. They are both nearly on par with the Mullard CV4003, just a touch brighter/leaner imo. Either would make a really first class input tube.
Just be patient they turn up quiet regularly.