Hi Qvart; Tomek is right that changing out tubes will change the sound of the amp but it will not change the amp's fundamental frequency response. The amp will sound different; maybe clearer highs, better balance and so on but due only to subtle differences in tube materials, mfrg process, and age. At the end of the day, nothing much will have changed.
This is a table of amplification factors for the 12A_7 family:
12AU7, Amplification Factor = 17
12AY7, Amplification Factor = 40
12AV7, Amplification Factor = 41
12AT7, Amplification Factor = 60
12AX7, Amplification Factor = 100
As you can see, subbing in a 12AU7 will reduce the preamp's total gain and, at the same time, provide more headroom but you may not care for the 12AU7's apparent lack of edge. But if you want to do a little tube-rolling, I'd suggest you start with a 12AT7; it will change the sound of the amp but not frequency response.
If this is your amp
Here, it's a complex, hybrid design using ICs, op amps, and transistors along with tubes. I'm sure there are other ways but if you want to change the frequency response of the amp, you have to change either (or both) the frequency response of an active component - like a tube - or mess with the signal characteristics.
The pics below are taken from the linked schematic. In the upper panel, that's V2B, the 'back half' of V2 - 12AX7 ahead of the Master Volume controls and the phase inverter. It has an unbypassed 1.2K cathode resistor that sets that section's bias. Adding a bypass cap; as small as .68uf or as large as 25uf will filter high frequency out of the signal. .68uf is a value commonly found in Marshall designs while 25uf is commonly found in older, vintage gear. Roughly, the .68 will eliminate some hf, the 25uf will eliminate more.
The lower half is the EQ section with several capacitors highlighted. These caps set the frequency range of the bass/mid/treble controls. As you know, messing with tone is a subtractive process; to increase bass, the controls subtract treble and vice versa. If these caps are board-mounted, it will be a PITA to get at them; particularly if it's done trial-and-error; you at your tech, he changes something, you play, you like/dislike, he changes, you play, and so on. If a change of tubes doesn't do it for you, you might want to consider the bypass cap. You can be there when the tech tugs the chassis, locates R18, and tacks a cap in place; you can play it right there and decide; more, less, or it's a big waste of time. Good luck with your amp! John