Clarinet advice:
1. I think the circuit and PC Board are solid. Although I ventured from the stock version as far as component selection, cabinet, and physical layout, it is still very much Jim Hagerman's Clarinet. If you are new to this hobby, follow the manual, use the parts Jim chose and you will be pleased. Go exotic, and be suitably rewarded with a component that can compete effortlessly with the most respected electronics existent.
One note on the filament voltage dropping resistor, R313 - 3.0 ohm. This gave me 5.80 volts ... a shade low. Changing this to a 2.5 ohm, 5 watt Mills brought this up to 6.18 volts, Much preferred. During this final "tweaking session" I also added a single teflon "O" ring to each of the nine pin tubes and rapped the pins of all the tubes with some Teflon tape. This is an effort to reduce vibration and resonance.
2. Give yourself space. You will always need more room than you think. Always!
3. Dress it up. Be it metal or wood, invest time in making it look as good as it is going to sound ... and it is going to sound great! I used "Front Panel Express" and recommend it highly. This gave my Clarinet a look of professionalism that I could never have achieved on my own.
4. Read this forum. Read anything by Tubesforever several times. He has taken this to eleven and his advice is good as gold. (actually good as teflon would be more appropriate) Keep in mind that just because something sounds extreme doesn't mean it's insignificant. Resistors, capacitors, controls, switches, wire placement, grounding, physical layout ... everything has audible effect to some degree.
5. Enjoy this hobby!