If you have a lot of the official releases I highly recommend an HDCD DAC, or HDCD CDP, makes a world of difference. When you listen to the HDCD official releases without HDCD they don't sound right.
Fortunately you can get the peak extension back using the software HDCD decoder. dBpoweramp can decode the HDCD code and so can CUETools. The software decoder though can't do much with the special transient filters that HDCD can do (I don't believe they've been able to reverse engineer them and even then I think the audio would need to be upsampled to be able to take advantage of them).
If you've got any official HDCD releases and don't have an HDCD DAC or CD player then give the software HDCD decoder a try. The decoder can let you know if the CD has peak extension.
Some of the HDCD discs don't have peak extension or transient filters. The only HDCD feature they seem to have is the special HDCD dithering. Yet the discs have the HDCD logo. Whether custom dithering is a useful feature worthy of the HDCD logo can be debated. But for those dics with peak extension the HDCD decoding is useful.