Well it looks like you have chosen a good volume and tuning point. I have it modeled in a 5 cu ft enclosure which if using 24" sonotube makes your internal hight about 19", tuned 23hz makes two 4" ports come in at 23" long. So there is no way you can get them to fit without putting a 90 in there which should be no problem. Come up about 15" with the elbow and you should have plenty of room for the rest to run horizontal. I am not the best at giving amp suggestions but the Dayton SPA250 is a good value and has the subsonic filter in there at 20hz to keep your cone excursion under control and should give you some nice solid output from 20hz on up that will blow the KLH away. You just have to make a little box for the amp or find a creative way to mount it to the sonotube.