I’ve had some experience trying out VAC+, NAL-NL1, NAL-NL2, and DSL v5.0 Adult on my miniRITE OPN 1. That’s because I have the ability to have up to 4 programs in my miniRITE, so I just put the different fitting rationales in each of the different programs so I can toggle back and forth between them for A/B/C/D comparison on the fly. It looks like for the IIC type OPN, it can only have 1 program, so you can’t really do A/B/C/D comparison on the fly like I can.
To me personally, and I suspect it’s different to each person based on their different hearing loss, the VAC+ sounds the most natural to me. But I do resort to NAL-NL1 in a challenging noisy environment where I struggle to understand speech. The NAL-NL1 gives me a sharper edge on the higher frequency sounds, and this helps me with better speech understanding. The DSL-v5.0 Adult also gives me a sharper edge to help with better speech understanding, but not as well as the NL1. The NL2 and the VAC+ don’t seem to have this sharp edge, hence they may sound “duller” for you, hence the impression you have with being in a barrel with these rationales.
I think the perception of how one likes a particular fitting rationale, beside how well it works for one’s particular type of hearing loss, there’s also a factor of familiarity involved. Since I was originally exposed to VAC+ first for a long time before I discovered that I could add in other fitting rationales in other programs that allows me to change fitting rationales on the fly, I might have tended to find VAC+ the most naturally sounding rationale to me, just because I’ve been more used to it, so that’s the “normal” way for me to hear, translating to being more “natural” sounding for me to hear.
In your case, on the other hand, since you’ve been more used to NL1 from the get go, it’s probably has become the norm for you, hence your preferred go to rationale. But I think it also does offer a sharper edge, at least based on my hearing loss, so that’s why the VAC+ and the NL2 sounds duller, more like in a barrel, for you. I think if you want more help with softer voices, the sharper NL1 is probably best at giving you this edge already, so I’d just stick with it if I were you. The soft sound perception control that VAC+ allows you to adjust basically does a similar thing anyway, because that adjustment allows you to select between varying degrees of “Comfort” (meaning softer tones) on one end, and “Detail” (meaning sharper tones) on the other end. So the NL1 is probably already giving you the most “Detail” setting by default in this rationale. So even if you were to switch to VAC+ to be able to boost up the Soft Sound Perception to the most “Detail”, it’s really already the same, or maybe not even as sharp, as the NL1 anyway.