4 ohm (or 6) Bookshelf speakers. Pass Labs 30.8

There is a better than average chance that the 30.8 and a vast majority of amplifiers both S.S. and tube will perform better with a higher impedance speaker. The impedance swing will be a less significant factor with S.S. than tube however. I think that you shouldnt worry about the impedance and concentrate on buying a speaker whose sound you like. 

mplstereo

I would worry too much as the Pass 30.8 gives more that you think, and it actually increases it wattage for each halving of impedance as well, so can do half decent current into low impedances.
This is the 30.5 which I take is similar just earlier version.
https://forum.audiogon.com/discussions/comparing-the-pass-labs-xa30-5-to-the-xa-30-8

While the XA30.5 may give 30Wpc into 8 ohms in class-A the distortion at this power level is low, at 0.015%.
The amplifier doesn’t actually clip (defined as the THD reaching 1%) until a much higher power level:
130Wpc into 8 ohms
195Wpc into 4 ohms
332W into 2 ohms single channel driven

Cheers George

The Dynaudio Excite X18 are 4 ohm and sound pretty good.  They have been discontinued, so you can find them around half price now at about $1000 a pair.

I also own the Buchardt S400s.   Right now using my Anthem MRX 720 as my amp... they sound quite good.  Sometimes exceptional, amazing bass for a bookshelf.  

Mads at Buchardt answers emails very quickly if you have any problems.  I bought mine just before they were swamped with orders.  I ordered the smoked oak, but took the option of dark oaks temporarily.  So I’ve had the dark oak since April with a promise of late July delivery for the smoked oak.  They really care about the customer and have great comm.  Mads has upgraded me to the spokes oaks as well.  I really appreciate their concern.

I’ve been enjoying these speakers since April.  Just sent and email yesterday and Mads promptly returned an email update.

They are still behind a bit on sourcing the smoked oak and hiring more people.  Apparently they have updated the speakers with some tweaks as well and he believes they have improved the speakers... so I can’t complain as they will send the updated speakers and have been able to listen to these long term... they are keepers IMHO.
They had them at Xponia but honestly they did not sound as good as they are with that setup in the room.

Personally think they did a disservice to the speaker.

I WOULD SEND AN EMAIL TO BUCHARDT AUDIO and ask them, their customer service is extremely good .....my bet is they would be able to give you useful advice.

i am planning on buying a different amp as the Anthem is for my Home theatre and plan sending an email to get advice from Mads Buchardt when I take that next step.

Dynaudio Contour 20 is really good but its a pretty big monitor and very current hungry. For something smaller and a little less demanding I'd be all over some Dynaudio Special 40.