People say they can hear the difference in capacitors. I have seen some studies that tends to support this idea, but it really isn't "solid proof" that you can hear the difference.
Some capacitors make a MEASURABLE difference. My Musical Fidelity XCAN v3 had some really cheap filter caps in the power supply; replacing those cheap ones with with quality capacitors with lower ESR made a slight but measurable difference in the the amp's S/N ratio, especially at the top end of the audio band. And, removing the DC blocking cap off the input and changing the output coupling caps to non-polarized parts with higher capacitance made a measurable improvement in low frequency response.
Coupling capacitors - capacitors that connect the preamp stage to the amp stage, or those that connect the output of the amp to the headphones- people say that electrolytic caps sound distorted when used in these circuit positions. The audio is flowing directly through these capacitors, and maybe people can hear differences in sound quality between electrolytic capacitors and polyester or polypropylene film capacitors, or oil-in-paper capacitors. Certainly these capacitors all differ in certain real, measurable properties like ESR and so on, and so maybe you'll hear differences. However, capacitors rated to handle the high voltages inside a tube amp and capable of the high capacitance values needed in coupling capacitors are expensive. It seems kind of silly to use $600 coupling capacitors in a $200 amp.
Bypass capacitors can also be important, some say, and most people like film capacitors in this application. It doesn't look to me like the stock Bottlehead Crack amp uses any - or has any need for bypass caps anywhere, though.
Power suppliy filter capacitors should be good quality, low ESR electrolytics, and it looks like the Bottlehead Crack uses good quality parts for this already.
So, I'd say, build the Crack in stock form. Then, watch the Forums to see what people do to modify them. See if anyone finds coupling caps that improve the sound. You could go out and BUY a few hundred dollars worth of exotic caps and experiment yourself to see if any improve the sound.... but I'd guess you have better things to do with your money.
For example, people in the Bijou D.I.Y. tube amp forum have found some oil-based motor starting capacitors to replace the electrolytic output coupling caps and they report that they sound better. These caps can bought off eBay from someone who is selling them to use with motors and doesn't realize that there are audiophiles out there who think oil-in-paper capacitors are "magic" and will pay silly prices for them.
Another tweak you can do is trying different tubes. There are tubes currently manufactured in Russia and China, and I think the Czech Republic too; maybe you'll like the Chinese tubes and hate the Russian ones. Also, there are "New, Old Stock" (NOS) tubes available on the market that have been sitting on some shelf since the early 1960's when tubes were still common. There were many manufacturers of tubes in the US, England, Holland and Germany in those days. You might find that the RCA tubes are great, or maybe Telefunken. Be careful, though, some tubes have gained such a reputation that the prices have shot through the roof- and so there are people out there COUNTERFEITING them! See http://reviews.ebay.com/Amperex-Bugle-Boy-12AX7-ECC83-preamp-tubes_W0QQugidZ10000000002395429 this guy explains how to spot certain fakes.