I don't have experience with DIY attenuators but the circuit looks very similar to the Weber attenuators. Basically a Rheostat to soak-up the power. I have one of these and it works flawlessly

https://www.tedweber.com/gadgets/attenuators

All of the writings I have seen is that you need to load your amp. So don't simply unplug your speaker and use your line-out/headphones. **That** will harm your amp. Otherwise the most likely scenario is to blow-up the rheostat you are using.

The Weber attenuators have a "safe" wattage and a "complete attenuation" wattage. Here is the Weber attenuation FAQ.

https://www.tedweber.com/attenuator-faq

Another "basic" attenuator is the Dr. Z Brakelite. It is rated to 45 watts and actually fits into your combo. Per the downloadable manual: 0 = 0dB, 1 = 2dB, 2 = 5dB, 3 = 8dB, 4 = 11dB

http://drzamps.com/product/brake-lite/

Here is a video describing the brake-lite. Interestingly, the dealer who made this video says during the video that virtually ever Dr. Z amp they are shipping has the Brakelite installed.

https://youtu.be/EVPqHZLdruo

Sorry I don't have a direct answer to your question. I think investigating the Weber and the Brake-lite will give you a good grounding (ha! pun!) on how the DIY one will work. I think the biggest danger is overpowering the rheostat that will burn-out.

HTH,