I think the XTZ Spirit speakers are probably neutral, and I really think they are good speakers. I will add too that it wasn't an easy decision sending them back. As you say, the Arendals are competing at a higher - so the comparison is not necessarily fair.
About Arendal and customer service, I must say that they have been excellent to deal with. They responded quickly to my questions before I ordered the speakers, and were equally fast to help me troubleshoot once I was experiencing issues. I have no complaints what so ever.
Well, I'm not sure yet. I have been looking at a few different models:
XTZ sub 8.17 (not available in satin black atm) or Sub 10.17
SVS SB-1000 or Pro version (not sure I like the ash finish, and the other finishes would not match the black Arendals very well)
I would like to keep it near 500€.
I have looked at the Arendal 1961 1S as well. Wife likes that option - but it is a little more than I would like to spend. Especially since I am in an apartment and will not be able to fully utilize it. This is actually my biggest concern at the moment - not knowing if or how we can use a sub in the appartment. This is one reason I have included the very small 8.17 above. Which of the above would be your pick?
If you go with XTZ then pick the Edge model cause it has the more reliable amp with 5year warranty. The 8.17 has 2years. SVS and Arendal has 5years. 10.17 Edge has small cab and some port noise issue are reported when listened loud some material with deep bass. This shouldn`t be issue with you. You would use it one port open most likely, if you want the rumble feel of ported sub. Check SVS SB1000 or Pro on youtube videos so you see the finish better. Arendal 1S would offer best performance, but side firing driver which needs 10cm clearance. It`s not exactly small, one member got it week ago and her wife was angry due to size of it. See the picture of dual 1961 1S below and show it your wife! My money would be on SVS/Arendal.