> I am wondering about the merits of going direct to the power amp.

I don't even understand why there's need to open up the amp.... Plugs and cables are invented just to avoid doing so....
Just use normal RCA/XLR out to power amp.

> Second question: any suggestions for good android apps to measure EQ in my room with my speakers. Or are phone microphones too rubbish to make that worthwhile?

There's an iOS solution. No android solution --- android simply has too many phones.
just buy a measurement mic (good ones are within $100) and you get precise measurements.

There are different possibilities to place ADI-2 DAC/Pro in a setup.
The following drawing gives you also an overview about
- where the D/A conversion takes place
- how far the Bittest can test for lossless audio
and some other information.
I personally regard it as very useful when the Bittest is able to test for lossless audio up to the D/A converter (connection types 1-4, see the red bullets with numbers). In your case you want to choose either 2 or 3.

It's easier to connect the ADI-2 DAC to the preamp of your integrated amp, connection type 2

I have an integrated amp as well (Accuphase E-600) and there is nothing wrong to connect the ADI-2 DAC/Pro to the preamp of the integrated amp. I get an excellend sound, thats it. By this you can still use the preamp section of your integrated amp to connect analog devices (note: the ADI-2 DAC has no analog inputs and the ADI-2 Pro only 1x analog I/O).
Could also be practical if you have a turntable and to use a phone preamp (if it's part of your integrated amp).
Also useful: you can use the volume knob of your integrated amp to control the final volume. Then you only need to take care not to overload the analog inputs of your integrated amp (with or without attenuators, see also below regarding attenuators).

When using the ADI-2 DAC as preamp, connection type 3

If you want to use the ADI-2 DAC FS as the only pre-amp for your setup it's possible. But as you say in your case you need to open the case and change a jumper position. Quick A/B or blind tests are not possible anyway and I wouldn't expect audible changes by that.
There are some fellow forum members who connected it exactly that way, but then you need attenuators most likely, so that you can use higher volume levels on the DAC to be able to make better use of "autoreflevel" (see below) without damaging your ears and/or equipment (because preamp set to 0dBFS with high reflevel connected to power amp can be extremely loud).

How to deal with level mismatches, switchable attenuators

The use of switchable attenuators can be useful to be able to set the ADI-2 DAC/Pro's volume closer to 0 dBFS in both cases
- connection to pre-amp: not to overload the inputs
- connection to power-amp: that it doesn't become to loud
with the intention to use the feature auto-reflevel. In this case the ADI-2 DAC/Pro automatically chooses the best suited of 4 reference levels (honoring B/T, PEQ, dynamic loudness settings) to optimize for SNR and dynamic over a range of 20dB.
The manual contains further information and an example calculation about the usefulness of "auto reflevel" in ch 19.3:
https://www.rme-audio.de/downloads/adi2dacr_d.pdf
https://www.rme-audio.de/downloads/adi2dacr_e.pdf

Here some general information from RME how to handle level mismatches with e.g. switchable attenuators.
https://forum.rme-audio.de/viewtopic.php?id=25399

Examples for switchable attenuators:
https://www.thomann.de/gb/jts_ma_123.htm (-10, -20, -30 dB)   €22
https://www.thomann.de/de/shure_a15as.htm (-15/-20/-25 dB)  €67
The JTS gives you a wider range, the Shure focuses on a more narrow attenuation range.

See also comment from Matthias Carstens, there is no better quality with the more expensive Shure, you can simply pick what appears to be more useful in your case regarding the attenuation range or simply choose whats less expensive to experiment with it, see: https://forum.rme-audio.de/viewtopic.ph … 66#p182766.

Also useful, TOSLINK switcher

What's also very practical, to put a TOSLINK switch in front of the ADI-2 DAC/Pro to be able to switch between up to 4 different digital sources (be it ADAT or optical SPDIF). I have found one which looks good, is meanwhile not too expensive, has a remote and with the Mk II model it even supports not up to 192 kHz: Oehlbach Optosel 4:1 Mk II.
Careful here, you need the Mk II product, the former claimed to support 192 kHz but only supported 96 kHz (wrong selection of TOSLINKs with lower BW inside). See e.g. here: https://www.amazon.de/Oehlbach-XXL-HDMI … S9ANK?th=1

Room optimization

If you have a PC or Laptop you can also use REW (Room EQ wizard) for optimizing your room.
https://www.roomeqwizard.com/

According to KaiS Audiotools by Andrew Smith for iOS is very useful and FFT and RTA are free.
https://forum.rme-audio.de/viewtopic.ph … 41#p178841
Maybe you can borrow an iPhone from a friend for this purpose.
https://apps.apple.com/de/app/audiotools/id325307477

You should first try to get the most out of it by clever positioning of the speakers. Acoustic optimization of the room would not be bad, but can reach certain limits quickly when having to deal with living rooms. The PEQ of the ADI-2 DAC/Pro can help to optimize the remaining critical areas, e.g. in the bass range at the listening position.

When using the PEQ. The last firmware changes gave you the possibility to see and use the two bands for B/T in the PEQ.
I personally would keep the two EQ bands for B/T for this purpose, to have the possibility for quick adjustments of Bass and Treble according to differences in mastering.