F700
Headphoneus Supremus
@mjm6 I hope I will be able to help you a bit, eventhough I have never listened to the ONP myself. First, let's start with the FR-graph from the 4 IEMs mentioned in your post.
ONP (from one the most trusted source for graph on the www)
O400 (the grey line applies from 120Hz, because of DF target compensated graph)
O800 & OV800 (posted earlier in the thread by @ryanjsoo)
Considering the years you've been spending very satisfied with the ONP, going for the O800/OV800 most likely will be a departure in terms of sound signature. The ONP graph looks a bit odd to me with (especially in the region between 400Hz - 4kHz) an elevated midbass/mids section and then a dip in the lower treble (2kHz - 5kHz). Not the most detailed IEM as it seems, warmish yet not fatiguing. I understand what you say about being able to enjoy your music for hours.
If you find most of the IEMs unlistenable, I don't think it's a good idea to directly jump on the O800/OV800. These definitely have more presence in the upper-mids. You might experience discomfort at first. Maybe your brain will adapt over time (or not), but it would be risky business to buy one or the other blind. Try to get a loan unit first.
Even if the graph suggests it, the OV800 are not bright or harsh to me. They are very musical all across the frequency range, a slight U-shaped curve that allows a large variety of musical genres to be played back to my liking. Always regarding the OV800, I love the switch system, because even if I usually let both of them off, the extra bass provides the right amount of rumble if the track calls for it. The mids on the OV800 are the star of the show. I rarely have listened to silky mids like that, with great texture and a welcome neutrality (meaning neither too forward nor recessed). Are you into vocals? If yes, the OV800 are a set to consider, while keeping in mind that the upper-mids might be the killer criteria for you. Finally, back to switches, having both activated makes the OV800 sounding more "V" than "U" and I don't like that setting. It's a bit too energetic in the highs, so I basically use two settings out of four available, the treble-boost never being activated. Please note that my source (Calyx M) is considered warm by many, but I find it just right and "analogue" sounding. All my IEMs sound good on this DAP, but the OV800 and O400 represent a special pairing with the Calyx M.
At a portion of the O800/OV800's price, you have the O400. These have the best sub-bass section out of any BA-IEMs I have tried so far. Impactful, tight, beautifully rendered and layered. Perfect transition into the mids, which also are great on the O400, enjoying the same attributes as the ones from the OV800. Still, the aforementioned big brother goes further here. The O400 are a tad warmer than the OV800 in general and have less presence in the upper-mids. Also, the treble is less extended and more relax on the O400, while very enjoyable and completely sufficient, if you are treble sensitive.
In a nutshell, the OV800 are more competent than the O400 technically, with better mids and treble extension. Level of details is approximately on par, soundstage goes to the OV800 (realistic presentation) while the O400 are not congested. The O400 have a better bass section and is maybe more coherent sounding.
My advice would be to try getting your hands (and ears) on the O400 and the O800/OV800 before any blind purchase (check with Marco from Oriveti, he is a very nice and helpful individual). If it's not possible, I would go for the O400 first. Based on the graph only, they should represent a step up from the ONP, pretty confident about that.
I hope I could give you some useful information.
ONP (from one the most trusted source for graph on the www)

O400 (the grey line applies from 120Hz, because of DF target compensated graph)

O800 & OV800 (posted earlier in the thread by @ryanjsoo)

Considering the years you've been spending very satisfied with the ONP, going for the O800/OV800 most likely will be a departure in terms of sound signature. The ONP graph looks a bit odd to me with (especially in the region between 400Hz - 4kHz) an elevated midbass/mids section and then a dip in the lower treble (2kHz - 5kHz). Not the most detailed IEM as it seems, warmish yet not fatiguing. I understand what you say about being able to enjoy your music for hours.
If you find most of the IEMs unlistenable, I don't think it's a good idea to directly jump on the O800/OV800. These definitely have more presence in the upper-mids. You might experience discomfort at first. Maybe your brain will adapt over time (or not), but it would be risky business to buy one or the other blind. Try to get a loan unit first.
Even if the graph suggests it, the OV800 are not bright or harsh to me. They are very musical all across the frequency range, a slight U-shaped curve that allows a large variety of musical genres to be played back to my liking. Always regarding the OV800, I love the switch system, because even if I usually let both of them off, the extra bass provides the right amount of rumble if the track calls for it. The mids on the OV800 are the star of the show. I rarely have listened to silky mids like that, with great texture and a welcome neutrality (meaning neither too forward nor recessed). Are you into vocals? If yes, the OV800 are a set to consider, while keeping in mind that the upper-mids might be the killer criteria for you. Finally, back to switches, having both activated makes the OV800 sounding more "V" than "U" and I don't like that setting. It's a bit too energetic in the highs, so I basically use two settings out of four available, the treble-boost never being activated. Please note that my source (Calyx M) is considered warm by many, but I find it just right and "analogue" sounding. All my IEMs sound good on this DAP, but the OV800 and O400 represent a special pairing with the Calyx M.
At a portion of the O800/OV800's price, you have the O400. These have the best sub-bass section out of any BA-IEMs I have tried so far. Impactful, tight, beautifully rendered and layered. Perfect transition into the mids, which also are great on the O400, enjoying the same attributes as the ones from the OV800. Still, the aforementioned big brother goes further here. The O400 are a tad warmer than the OV800 in general and have less presence in the upper-mids. Also, the treble is less extended and more relax on the O400, while very enjoyable and completely sufficient, if you are treble sensitive.
In a nutshell, the OV800 are more competent than the O400 technically, with better mids and treble extension. Level of details is approximately on par, soundstage goes to the OV800 (realistic presentation) while the O400 are not congested. The O400 have a better bass section and is maybe more coherent sounding.
My advice would be to try getting your hands (and ears) on the O400 and the O800/OV800 before any blind purchase (check with Marco from Oriveti, he is a very nice and helpful individual). If it's not possible, I would go for the O400 first. Based on the graph only, they should represent a step up from the ONP, pretty confident about that.
I hope I could give you some useful information.