We have one Apollo 8Pre and the new black Apollo 16 on loan to see if we are gonna upgrade to the new ones from the Thunderbolt ones we have today and in all honesty, we recorded two sessions each on both of them and then we listened with blidfold on all of us (9people) used to the best sounding gear and it was hit and miss there
Guesstimates and those went both ways, one sec the older won and the next the 8Pre and we had both the older 16 and the new 16 connected to both Apollo Quad units and taking turns leaving the room to set them up for the rest of us so .... Verdict:
This is OUR OWN OPINION and should NOT be a benchmark to buy or not to buy, just what we found and The new Black units do have a ever soo slightly better preamps, we found them to be very slightly more open and cleaner and the 16 units were toe to toe frankly.
So paying like $2000 for a used mint Apollo Quad silver with Thunderbolt 2 card expansion or $3000 for the new one : I will stay with our silver units, both the Apollo Quad and our Apollo 16 both TBolt2 and wait for the prices to go down in a year or so.
We used the following gear testing both old and new Apollos:
Mics straight into the Unison on the Quads with flat input, no plugins or preamps except for the ones in the units.
/ Manley Ref C made the refordings very sweet sounding and full recordings
/ ELA M 251E made the recordings even better and more bottom end and cleaner highs with perfect midfield sound.
We also tried the same signal path with a Rφde Classic 2 which was a waste of time since those mics are IMHO very crappy sounding and overpriced to say the least. Sharp harsher sound and we had to push the preamp input a lot to get a even signal to the more epensive mocs.
Also tested My fave mic for the price Telefunken Copperhead which was a winner for all the mics except the ELA M251 of course
And lastly we did all these setups with the Neve 1073 Unison and The API unison channelstrip and tried a lot of settings with LA-2A and 1176 and also a few of the UA preset settings for Unison since we have all of the UA series plugins and they sound amazing and can't compare to lots of other DSP plugin powered interfaces.
... and I have to say in closing that it is worth that extra $1K to get the new pre's if you don't have a few great outboard preamps and or Console and yu are gonna use the unit for recording straight into it and not go thru anything else.
We have a lot of fine outboard gear and our trusted SSL 4000G+ and even though the outboard gear sounds wonderful I really am impressed with the Apollo sounding boxes and their Unison Funtion.
So for those who usually skip the "fineprint" and jump straight down to the bottom of long posts like I do sometimes:
The new Apollo Quad 8P have better sounding preamps even though just barely noticeble and IMHO worth the extra money if you don't have a lot of options in the studio. If you have loads of nice pre's and comps etc I would spend the difference on more gear rather than the slightly better preamps
Best of Luck and best would be to get one on loan and put it thru its paces.