I take it from the tone of your post that you are unhappy with the new tubes. It may be that you were sold tubes that are worn out or are fakes. Some people that claim to sell NOS(new old stock) are actually selling WOOF (worn out old fakes).
The other points above are valid. Let them cook for a while and see what happens. The designer of your amplifier may have chosen a particular brand of tube because it has some characteristic that works well in his design. For instance, maybe the originals are on the high end of some spec like input capacitance or gain and the NOS tubes are on the low end. These tubes would sound very different and the NOS might sound worse.
The other points above are valid. Let them cook for a while and see what happens. The designer of your amplifier may have chosen a particular brand of tube because it has some characteristic that works well in his design. For instance, maybe the originals are on the high end of some spec like input capacitance or gain and the NOS tubes are on the low end. These tubes would sound very different and the NOS might sound worse.