Technically, any integrated amp will do. You might also end up with feature duplication but if you already own the device you didn't waste money, at least. If your integrated amps don't support digital inputs you can be very sure that no internal digitisation and DSP (as mentioned by
@VintageFlanker) will be performed. Even if they have digital inputs, the analogue inputs will stay in the analogue domain in most cases (but it might be worth checking).
Stuff like WiiM's EQ or room correction can only be applied to the Ultra's own inputs, of course, not to any of the extra inputs the amp might provide. That's probably trivial. If you want to use WiiM Ultra subwoofer management you must use it's own volume control. Otherwise the relative level between sub out and mains out would constantly go out of match as soon as you change the volume in the amp. You would have to settle for one setting of the amp's volume control, match the subwoofer level in the WiiM Home App and stick with the Ultra's volume control from that point on.
Some users worry about relying on a digital volume control, for various reasons.
One reason is sound quality, since digital volume control is believed to be lossy. Well, it is in theory. In practice pretty much every DAC supports a higher bit depth than the signal it is fed. The WiiM Ultra is no exception here. The DAC supports 32 bit resolution while (I believe) WiiM actively limits input to 24 bit. This leaves so much headroom for volume control that it effectively isn't lossy even down to the lowest levels where your hearing will not able to tell 24 bit music from 10 bit music, anyway. This comparison (24 bit/10 bit) is not based on scientific evidence, I just made up an example to illustrate my point. There are other techniques used in digital volume control chips to increase the effective resolution where it's needed but I don't know if WiiM makes use of that or if. In practice I cannot heare any sonical imperfections in digital volume control with my WiiMs and my Lyngdorf digital amplifier compared to my pure analogue gear.
Another point is the problem that volume control purely done by software is potentially prone to software errors or unexpected software features. An external protocol like e.g. Chromecast, Tidal Connect, Spotify Connect can have control of the volume setting when switching to the related source. Therefore it could happen that the Ultra used as a pre-amp suddenly plays at full level, potentially damaging speakers or human ears. Bugs in the device software could lead to the same behaviour. Some prefer to have an analogue volume control (e.g. a passive potentiometer or an analogue pre-amp using one) inserted between the Ultra's analogue output and the amplifier's input as a safety measure, so they can limit the maximum output, irrespective of software behaviour. This can never improve sound quality, but only degrade it, potentially, but yes, it does add a level of protection if you are concerned. What I said about subwoofer integration holds true here as well.
Personally I've been running the Ultra as a pre-amp for weeks now, currently directly feeding a set of V3 Mono power amps (not F3, sorry, that was a typo) and there has been not a single issue.