Are higher end preamps worth the investment when you are only streaming Tidal & Spotify ?

First, try Qobuz vs Tidal, if available to you.

To most, Qobuz sounds better than Tidal and way better than Spotify.  I only stream, and would love to make a new system without a preamp.  But, so far, my systems sound better with a quality preamp.

Only a waste if you don’t massively upgrade your streamer and DAC.

I tried DAC direct and it lacked drive and dynamics ,  after owning that Zesto it was clear where all my systems in the past were lacking.   

Installing that elevated my entire system to another level.  Also, it demonstrated how good my Quicksilver monos really were.  

When I connected it to my 300b it really let that amp shine.   A good pre is crucial to good sound.  All sources pass through and it drives your amp.   Most overlooked component in many systems.  

I sold that Zesto to fund a DHT preamp and am using an old Sonic Frontiers preamp.  A temporary major step backwards in sound quality.  Sounds good but no where as good as with a great preamp.

Why not consult with Dan Wright? He can send you one of his preamp or the newest Analog Bridge to try it out. Whatever he suggests, it will be a great match for your KWA-150. 

I agree with others, a high quality preamp is indispensable. 

A preamp is little more than switch with additional tonal insertions. Assuming that the highest 'fidelity" is "straight wire with gain" why add an unneeded switch?  It might sound "better" but cannot possibly be more "fidelitous."

I've had Tidal and Spotify. Dropped them for Qobuz. It DOES sound better to me.