Once my system was no longer entry level and I was at the point in which the speakers I owned I considered my final pair I started what I called the final upgrade circle. I then upgraded the piece of equipment that was lowest in both cost and where they sat in that brands line. Yes cost can be a determiner. The higher you move the more you must spend to hear significant jumps. Eventually you begin to hit your maximum dollar amount you will want to spend on individual components and this starts that last circle that will level them all so hopefully one component does not hold your system back. The most interesting were the amplifier and preamplifier upgrades, My last two components that needed to be upgraded. These upgrades was where I realized how incredibly important those two components were. They really brought the sound to a level I never thought I would have been able to obtain
With all the above said some one mention the preamplifier partially due to its difference in cost to the amplifier. I agree with that analysis and it was proved to me when I went out on a limb and purchase my first to me really expensive preamplifier that was significantly more expensive than my amplifiers and heard what a difference it made. I believe if you are near the end of your upgrading purchase go for the preamplifier before the amplifier.
The only thing at this point that will be upgraded will be my DAC and streamer and only due to some unknown changes in music delivery and technology changes in those areas. This is not to say I have the best system around but I hit my limit in what I can spend and am very satisfied with the system’s sound. Took years like most people.