sir, do you have a 7.1 speaker arranged of your HT room na pala? if yes, then you are already used to hearing the FL+C+FR+SL+SR+RSR+RSL (rear surround left + rear surround right) audio effects when source is either TrueHD or DTSHD-MA. awesome surround bed effects, di ba?
with virtual height up-mixers (Dolby Height Virtualization or DTS Virtual X) when movie audio source is Dolby Atmos or DTS:X, respectively, and you will re-use the same 7.1 speaker arrangements (without actual height speakers), then you will benefit from these audio upmixers to "virtually hear" sounds above you. this will be more awesome surround bed + virtual height effects.
the absence of physical height speakers will give you virtualized heights like when there are audio coming from above (airplanes/helicopters, thunders, rains, etc.), you will actually hear them like they are really coming from above.
x2500 or x2600 are both native 7.1ch amplification anyway. so if you'll re-use your 7.1 speaker arrangement/configuration, you will benefit more from x2600's virtual height upmixers -- that is, without moving your rear surround speakers on top of you. with x2500, you will be forced to move your rear surround speakers and configure it as height speakers -- to maximize with the ATMOS/DTS:X immersive surround effects ("sound all-around you", including above and below daw) hehe
at least you have 2 options for each model:
Denon x2500
- move your rear surround pair above you and set them up as height speakers. you will enjoy 5.1 audio bed effects + 2ch height effects
or
- retain the same 7.1 arrangement and you won't have actual height speakers. you'll enjoy the usual 7.1ch audio bed effects na nakasanayan mo na BUT without height effects.
Denon x2600
- move your rear surround pair above you and set them up as height speakers. you will enjoy 5.1 audio bed effects + 2ch height effects
or
- retain the same 7.1 arrangement and still able to maximize "sound all-around you", including above and below kahit "virtual lang" using the virtual upmixers. in this case. you'll enjoy the usual 7.1ch audio bed effects na nakasanayan mo na + virtual height effects.
when you have ATMOS/DTS:X sources, it would be best if you have actual height speakers set up. 7 narin naman na speakers mo eh, lipat mo nalang sa taas yung 2