With an amp input impedance of 100K you have a lot of options. Firstly I would recommend that you browse back through this thread and find the diagram that shows how to insert the caps into the opamp board sockets. Basically you remove the opamp board and insert the cap legs into the left hand socket. Doing the mod this way makes it easily reversible.
Next I would recommend that you familiarise your self with the cap value calculators
HERE. Looking at the calculator on the right, you need to enter your input impedance in ohms (100000) and then decide how low you want your bass to go. I'd recommend that the highest value you enter in the Hz box is 10Hz. You can go all the way down below 1Hz if you like but there's not much point. There's more info on all that earlier in the thread. So if you put in 100000 ohms and 10Hz you get a cap value of 0.16uF. That means you need to look for caps that are 0.16 or higher. However there's not much point going above 22uF as that will give you a bass roll-off frequency of 0.72Hz. Don't worry if you can't find the exact value of cap recommended by the calculator. A close value will be fine.
Now that you know the cap value you need to decide the cap type. You mainly want to concentrate on non-polar caps, sometimes called bi-polar. Electrolytic caps are the worst type for this application but they're the smallest and cheapest and some do sound pretty darn good. There aren't many non-polar electrolytic's. Seen as you are able to use low value caps I'd recommend you look at film caps like polypropylene, polyester, teflon etc. They all sound a bit different. The only downsides are that they're bigger and more expensive. Teflon's are some of the most expensive but arguably the best. Some people like oil impregnated caps. These can be good but pricey. You would not believe how expensive some teflon and oil caps get. I'm talking £1000+ for some crazy ones.
On the subject of voltage ratings; anything from about 6V upwards is fine. Some film caps have ratings over 1000V which is also fine. Just remember that the bigger the cap, the bigger the cap leads. Leads above about 0.8-1mm probably won't fit in the sockets.
The choice of caps is mainly down to the price you want to pay and whether you want to be able to replace the cover or not.
THESE are some of the best electrolytic caps and are a great entry point to see if you like the passive sound.
THESE film caps are highly regarded and are good value.
ClarityCap's are also good value, as are the
Mundorf Supreme. Elna electrolytic's don't seem to work well in this application, so avoid them.
Another tip is to avoid branded caps from foreign countries on eBay. There are lots of fakes around.
Happy cap rolling.
Originally Posted by HighFidelityGuy
With an amp input impedance of 100K you have a lot of options. Firstly I would recommend that you browse back through this thread and find the diagram that shows how to insert the caps into the opamp board sockets. Basically you remove the opamp board and insert the cap legs into the left hand socket. Doing the mod this way makes it easily reversible..
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Any bit of old abrasive paper will work, but take a bit longer. Last resort, go outside and rub them on a brick or concrete paving slab - anything abrasive will do, and you're not too worried about a perfect finish for this... 
