There will be a slight difference as all tubes are not created equal and they all sound different. That said however, you notice much more difference in sounds the closer you get to the input of the amp, and the phase inverter tube is a ways down that chain. Old stock Westinghouse is on par with old stock Tung Sol and the 6AN8 wasn't a tube designed specifically for audio use, so aside from possible hum and noise problems from one tube to another, there won't be a lot of difference. Use it if it works well, and if not, find another tube to use basically. NOS 6AN8A's are pretty cheap.
Greg
