Couple of thoughts …
If you’re displeased with the quality of the AIFFs you’ve purchased – stop buying from that source. I know this is blatantly obvious, but really – trust your ears and vote with your wallet. I’ll repeat: trust your ears.
I wouldn’t place much stock into the assessment of ‘old’ music as many older recordings were mixed and engineered exceptionally well – Rudy Van Gelder anyone? but, production technics change over time and much modern music is often, not always, over compressed and you may perceive a difference in overall loudness from older songs as opposed to current songs.
I became displeased with the quality of the VBR MP3s from my eMusic subscription (obviously low quality and sometimes unlistenable.) After many complaints, they improved upon the quality of their offerings. Still not to my satisfaction, I throttled back my subscription. My solution was an iTunes Match subscription where I replaced those dodgy files with standard quality AAC files, but I’m well vested in the Apple ecosystem and have no (significant) issues with iTunes.
You may want to acquire a tack, or two, from another source or rip from a proper CD yourself. Bare in mind that not all CDs are mastered equally. It is possible, if unlikely, the source of your AIFFs was not well mastered as opposed to a another reissue of the same track. I’ve been listening to a Greatest Hits collection from a widely popular 70’s rock band and while I enjoy the material – the CD sounds awful. I think I’d rather hunt down the individual favorite tracks and buy them individually from the remastered editions … not that all remastered editions improve on the original either, but that’s a conversation onto itself. So, agin – trust your ears.
Regarding the comparison of two tracks, here’s an article from SOS on how to go about it (the article is specific to Logic Audio, but the principles can be applied to any DAW. http://www.soundonsound.com/sos/mar11/articles/logic-tech-0311.htm
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