I don't like this scheme very much...
For line out I'd use 500Ω and 4k7 resistor.
For speaker out you should go for a L-pad attenuator and use any tool online for vlues calcs. Here is one http://www.sengpielaudio.com/calculator-Lpad.htm
Pay special attention to power in each resistor because it WILL get HOT and it have to get a lot of power to heat. If your amp is 100W depending on the attenuation wanted you should go for more than 100W resistors. If you want 20dB atten (1/4 percived level, I think it's nice for a bassman full up for confortable listening) resistor will be handling 81W when your bassman puts 100W, if you want to saturate output stage it will be putting more than 100W and mos't in R1 of the scheme in the web, so it should be 200W at least... Could be 4x50W in series-parallel. (I don't know if it could be 100 2W resistors but it will have some inductance that may affect if layout is not well worked)
Any case you should be specting a lot of heat and a massive heat sink and probably a fan will be necesary.
JS
EDIT: I often recomend lower sensitivity speakers and less speakers but hard to get a guitar speaker that handle 200W... (I'd love to build a SE amp, something like AC4, with an 10' alnico eminence to recording at home really hot with low levels)