External interfaces can be convenient. They have better digital-to-audio conversion, better headphone amps, better drivers, and nice analog controls. The fast track pro you mentioned is a perfect example. Others are presonus audiobox usb and focusrite 2i2. The behringer uca202 works in a pinch (very good drivers) but has fewer features (for example, only unbalanced in/out).

If you are not planning on getting a subwoofer, the 8 inch monitor will be much better. My recommendation to anyone using studio monitors is to a get a subwoofer, though. Even 8-inch monitors are quite sad on the low end and can make the playing experience less satisfying. Don't believe it when monitors say they reproduce sounds well down to 50 hz or whatever they have listed. Your ears will disagree.

In another thread, bfb, who owns both the Hs80s and the KRK RP8 monitors and uses them with his piano said the Yamahas probably have greater accuracy (i.e., for mixing), but the KRK's sound nicer with the piano. They are around the same price so you might consider looking that direction. And if you do get a subwoofer, you can save money and probably increase quality by getting the smaller speakers. Some 8-inch woofers have a hard time with mids.

If I was buying a studio monitor right now I'd get a pair of KRK rp6 and then the KRK 10s.

Krk is pretty solid. The Rokit Powered line is their lower end product, but they are, in fact, the most popular studio monitors in the world in terms of number sold. Yamaha's a great name and they made that classic old style studio monitor that sounds horrible but makes great mixes. Theirs are better for mixing than the KRK, but we aren't really mixing here. (at least, I'm not)

I think the fact that the KRK's are not made of kevlar actually makes them sound less harsh than other monitors (a common problem). Their higher end stuff is kevlar, and probably has many characteristics that are superior, but according to their marketing, it takes a lot of work to make kevlar not harsh. That's just marketing, though, so take it for what it's worth.

I had the krk rp8's and they were only ok by themselves. I wasn't happy with them but at first I wasn't sure why. I decided to try the woofer and liked them much better after that. So in my opinion, yes, the woofer is very helpful, though it is very expensive also. If you are short on cash, you can just get the monitors and then upgrade later when/if you feel it's worth it and you have the moolah.

It will work ok with no sub, and it will even sound ok. Just not as great and complete as it will with the sub, if that makes sense.

It sound to me like you have a good plan. My advice is to start with the RP6's. They don't have quite as much bass but they are better in the mids and they are cheaper. And they aren't horrible in the bass.

It is a bum deal that DP's cost so much to really get right. But luckily it can be done a bit at a time. At the moment I have NO speakers for my piano. Just headphones. Don't be embarrassed to do the same if you don't urgently need speakers, haven't decided which ones are really right, or don't have the cash to throw around.

Is there a large difference between them?

Krk is pretty solid. The Rokit Powered line is their lower end product, but they are, in fact, the most popular studio monitors in the world in terms of number sold. Yamaha's a great name and they made that classic old style studio monitor that sounds horrible but makes great mixes. Theirs are better for mixing than the KRK, but we aren't really mixing here. (at least, I'm not)

I think the fact that the KRK's are not made of kevlar actually makes them sound less harsh than other monitors (a common problem). Their higher end stuff is kevlar, and probably has many characteristics that are superior, but according to their marketing, it takes a lot of work to make kevlar not harsh. That's just marketing, though, so take it for what it's worth.

I had the krk rp8's and they were only ok by themselves. I wasn't happy with them but at first I wasn't sure why. I decided to try the woofer and liked them much better after that. So in my opinion, yes, the woofer is very helpful, though it is very expensive also. If you are short on cash, you can just get the monitors and then upgrade later when/if you feel it's worth it and you have the moolah.

It will work ok with no sub, and it will even sound ok. Just not as great and complete as it will with the sub, if that makes sense.