implementing gain will increase the loudness and is good to have for more power hungry phones.
it good to be able to decrease gain for use with sensitive phones and iem, with some amps they start you off at very low volume like the matrix and let u increase the gain to 20 + db.
others may start off in the middle and let u tweak + or - it varies a lot from different deigns.
it also varies how sensitive each person ear is as well so just because i think the sensitive denon is super loud on the hdp, dosen't mean u might, but i would get others opinion who own both on these matters, but let your own ears decide. i have heard reports of others having to crank the hdp, and some i think were referring to there denon. i did not expect this with the hdp. it makes total sense with cans like the hd650 or beyers, but not denon imo ymmv.
for rocking out hdp does a good job surprisingly with the denon, yes but not for monitoring, tv, or just really long listening sessions, i have and do and am forced to keep the pot near 0 on the hdp.
having said that the hdp sounds really good, and i think my vinyl sounds better on it than on my matrix m stage, the hdp has a lot of sparkle to it, and it surely is not thin, it makes others sound thin, and is really great for rock n roll, and it is very loud on certain phones and iem i only wish it had a lower and higher gain setting. when i bought it i had no intention of listening to its head amp, but now whenever i listen to a really good lp i always like to hear it again on the hdp as it sparkles more, is clear and more lively and energetic, everyone should listen to some vinyl with their hdp seriously
wow i hoped that helped and doesn't complicate.