If your whole system costs $40,000....

I'd estimate my main system (digital source included only) cost about 15-17K retail.

The cost to me was about 8-9K.

My main digital source is computer music server, DAC, + Squeezebox Touch, total cost ~ 1.6K with main and backup disk storage.

That's about 10% of total retail cost.

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I see you just don't get it. Trying to peg a percentage to a component is an exercise in futility. As far as retail my system is probably 60% speaker, 15% amp. 15% turntable/cartridge, and 10% digital source/preamp. How did that help you?

I was trying to answer what you initially posted and the only way to even begin to help is if we know what you have ....

I have been wondering my CD section is the weakest link,.

good luck
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Excluding speaker cables, ICs, and power conditioners et al, I have three systems that meet your criteria. For the CDP in each, the cost percentages are 8.2 %, 11.4 %, and 22.1 %. Figures are based on retail costs for new gear and what I actually paid for vintage gear. The CDPs were all purchased new; two of them are current models.

Survey:

"how much will you invest in CD player with that sum?"

Answer:

As much as it takes so that I enjoy listening to the music thru it. It so happens that at this time, it's around 10%.

I would consider it foolish to limit myself to a certain % of say, at or about xx%. I once put an extra system together a few yrs ago that paired a $900 2a3 SET integrated with a pair of %550 single driver speakers. Those are "msrp" prices, not street or used. The CD player was $3000. That works out to 200%. I really enjoyed it. I doubt I would have with a $150 (10%) player.

Personally, I would never spend more than about $2000.00 for a new CD player, no matter how good the rest of my system is (I assume you are using cost as a proxy for quality -- always a risky assumption in this hobby). I've had any number of CD/SACD players (I bought my first CD player in 1984 or 85, I'm not sure which, at the "breakthrough" price of $400.00), with purchase prices all over the map. My experience has been that once you pass $1500/$2000 MSRP, the incremental increase in reproduction quality is so ephemeral, and subjective, that it doesn't justify the expense. I wouldn't be too surprised to find that we have folks with $40K systems out there that are happily tooling along with $700 or $800 CD players.

YMMV