I don't use ext tubes, but I do have a Raynox 250 close-up diopter. It delivers surprisingly good IQ and at around $50 it's good value. It works well on the 75 - certainly well enough for casual macro work.
As there is no glass in these extension tubes, there should be no IQ degradation. Lots of generic brand extension tubes out there at different price levels. Can't tell you which ones are good, as I have not used any, but you can get more info here: