??? How Much Would You Spend ???

Speaker Selected First = Make a provision for the Signal received to be turned into an end sound that is the best.

Thereare dependencies on all that is upstream and the ambient of the environment to Control this method. 

Source Selected First = Signal produced and sent is the best one can have created for their budget.

This is my choice for a Audio System, 

Only thing I have learned in 50 years is never spend more than 75% of the current speaker price on any single component ahead of them. The one exception would be for vinyl-based systems where the whole phono chain is considered a single component. So considered, you might pay more for the phono chain than the current MRSP of your speakers.

Tough to find a consensus because we have different budgets and spending priorities - those with larger budgets would usually spend more than those on a more restricted budgets. Rough guess those on a tight budget 3-7k for electronics (pre+amp) and those with loose wallets 10-15k+. Above this amount speaker upgrade becomes an attractive budget alternative.

Amplifier is the most important component after the speakers. At least 50% of the cost of the speakers should be invested in an amplifier and you have to not only look at cost - matching amp to speakers is critical. Rarely any other component will make as much difference as a carefully matched high quality amp.

Systems are called systems for a reason so they should be built holistically - a chain is only as good as it’s weakest link etc.