Speakers to pair with Technics SA-C600

[FBech]

Not sure why it has to be standmount speakers if you cannot afford the stands to put them on....
However both of those you mentioned should work well.

I am placing them on a flat furniture with similar height than the standers, with the Technics unit in the middle

 

[Fandango Andy]

I am placing them on a flat furniture with similar height than the standers, with the Technics unit in the middle

I recommend floorstanders as you said in a previous response you said you had space for them. It now appears you are looking for bookshelf/standmounts.

Therefore I will revise my recommendation: Triangle Borea - BR03. They are less expensive than the floorstanders so within budget. They still fit your room size. Made in France and widely available all over Europe. You can return and get money back within 30 days so can try them out without risk.