You know, you were saying some of your mp3 files sound flat... you can compare them back to back by simply re-burning the mp3's back to an audio cd. When you make an audio cd it takes the mp3 and re-encodes it to .wav but it retains the sound. It can't recover the data that was lost in the compression.
When I was going through the big internal debate that you are, I took 10 second song clips in .wav format (uncompressed, straight off the CD) and then copied each one and converted it to .mp3 using 320 VBR. I then recorded the .wav and the .mp3 of each song back to back onto an audio disc.
I have a very nice system in my convertible and I put the disc in, in my garage, with the car off so there was nothing but the music. I can honestly say the difference is completely transparent to me. Any artifacts I thought I heard on the .mp3 I'd go back and sure enough, there they were on the .wav. Any distortion or tinny cymbal crashes, identical in sound to the .wav file.
When I made my disc, I used a wide variety of genres but I picked tracks that focused on treble, with bright horns and lots of cymbal crashes. The reasoning for this is, mp3's mostly cut off treble, and the higher the bitrate the higher the frequency spectrum that is retained. From what I understand, 320 bitrate cuts off at about 18 khz for the most part, which quite frankly is right at the upper end of what humans can really hear anyway. Sure 22khz is the official number, but only like 5% of the population can actually hear in that frequency range. My ex wife couldn't hear the flyback transformer on our TV (I could) and it operates at 16khz. Treble was completely lost on her.
So give yourself a true honest side by side test, and if you want to make a blind test, have someone else pick the order in which they're burned so that you have to guess which is the mp3 and which is the .wav. If you can accurately pick out the .mp3 more than half the time, I'd say FLAC is a good call. if you can't, you're wasting your time, just use mp3.