I regret buying an AKG k7XX, for the price that they were the build quality and tonality was awful. I have since sold this for a tenth of price i bought it for.

I also regret buying an AudioEngine D1 DAC as that really didn't make a difference in sound quality compared to my onboard audio and didn't really offer any more volume. Although the optical input was handy sometimes.

Apart from that don't have any other regretful purchases.