Atmos would require additional speakers and the drivers in my speakers have been out of production for some time so making new speakers that match would be difficult. Is the TrueHD etc. a huge leap forward in sound quality over the Dolby Digital/DTS-ES that I currently have or is it just a smaller increment?
I can't reliably pass a blind test when comparing regular Dolby digital or DTS at 640kbps and 1500kbps to lossless, and most of the time I can't even pass at the DVD standard of 448kbps when actual movie content is involved. Multichannel compression is much more efficient than stereo since there are multiple channels to "borrow" bits from. Unless you want discrete 7.1 I'd say no.
I'll also say the new Dolby surround matrix upmixer is a massive improvement to Prologic II, even with a 5.1 setup, it's nearly as good as discrete surround, so if you're big into upmixing music or stereo content I'd say a new receiver is worth it just for that.
Also, heights don't have to be 100% timbre matched, I've found so long as there aren't major differences in tonality, (for example, you wouldn't want bright floor level speakers and heights with subdued highs) anything will work well. Being that you're big into diy you could easily design speakers to have wide dispersion characteristics necessary for facing the speakers down and getting good coverage across the seating area.
Atmos is the largest improvement to multichannel sound since discrete 5.1 and really is worth the upgrade. You may not be interested in it, but I guarantee if you ever get to hear a proper atmos setup you'd instantly change your mind. It's less about just having cool aircraft flyover effects and more about having a believable three dimensional sound bubble that gives "space" to the soundstage. It's like going from stereo to surround, no matter how good a stereo mix is its very two dimensional and can't accurately reproduce a 3D soundstage.
Even with legacy mixes, both Dolby surround and DTS: Neural X (the latter seems to work best with multichannel content) do an amazing job figuring out what to remap to the heights.
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