There's no reason you can't do it, but here's a few things to bear in mind:
1) Cost of tooling up?
The chassis will cost money, and you'll need to cut and drill, possibly tap screw threads also. If you already have the tools that's fine. Otherwise it pushes the cost up, so include this in your calculations. you may save less money than you expected.
2) Where are you getting the PCB from?
If you plan to make your own PCB add more tooling-up cost; if you plan to buy, be aware that some PCBs on the market have poor noise/hum performance. A poweramp is very simple though, so point-to-point wiring is probably a better way to go anyway.
3) Layout
Making all the connections in the schematic does not guarantee good performance -- good physical layout of the components is needed or you could end up with a humming, noisy, shitty-sounding bastard of an amp. This is true even of solid-state amps, and tube amps are more sensitive to this because the impedances are much higher. OTOH, get it right and it'll probably outperform many commercial amps in this respect. Just be aware this may take more time than you expect.
4) No guarantee.
If you wire it up wrong you'll need to replace blown parts and start again. Burn out your output transformer and your savings shrink pretty drastically. Exploding electrolytic caps aren't funny either. So be careful and make sure it's solidly built, or a lot of time and money could go up in smoke.
5) Do you plan to use it live?
What if the stage manager refuses to let you plug it in because it isn't CE marked? No CE mark means no safety testing and no electromagnetic compatibility testing. The venue's insurance might be void if they allow uncertified equipment to be used, so it'd be a valid objection.
6) Insurance
If your amp gets eaten by buffalo, can you spare the time to build a replacement? If a commercial amp is trashed you can wave your credit card and get a replacement more or less immediately, and if it's insured you won't have to eat nothing but pasta for 6 months as a result. Trying to insure a DIY poweramp? I've never tried, but it could be tricky.
7) Safety
High voltages can cause permanent injury and death. If you built the equipment causing it, you are legally responsible (unless you died). If you bought the equipment and used it as specified you're not.
None of these considerations are deal-breakers, but you should give them a bit of thought before committing yourself.
Best case scenario is you'll have a high-performing amp which will survive a bazooka attack, for low cost.
Worst case is you'll wind up with a shitty-sounding amp that cost more than expected and ate a whole lot of time, and which soon shorts out due to a mechanical shock dislodging a loose screw and burns down your house or something.
One final caution -- you may find yourself sucked into the wonderful world of circuitry, and wind up spending way too much time with a soldering iron in your hand instead of making music.
