Yes you may try, but use one pair in V8 and V9, the other pair in V7 and V10. Always check your bias when putting new tubes. If you have an 8ohm speaker load before (assuming you matched when you decided to run on one pair 6L6GC only), you should have a 4-ohm load now if you want to maintain a matched impedance. On fenders you can mismatch 2:1 down or up but keep your eye on your plate dissipation.