Nobody wants to wait to enjoy music. If you can spend $300 today and keep your sanity, you should do it. If a year from now you have $1000 to get something even better then awesome! But you wouldn't have had to go a year not enjoying music.
A few more questions- have you tested your sound card with anything else? And which sound card do you have, or is it motherboard integrated audio? Easy way to find out is look under "Sound and Audio Devices" in Windows Device Manager. There will be videos online to show you how to do this if you're not familiar. I'm also assuming you tried different cables from the computer to the receiver already.
Do you have friends from whom you can borrow another receiver? Seeing that yours is 30 years old I'm sure that probably is where the problem lies, but it's best to be 100% sure before spending money.
As far as speakers go, trying to redo the crossover to accommodate the MTX sub would be a total waste of time and money. Sell them for what you can, and cut your losses.
I know you are fixated on
tubes (edit) but I suggest getting your system to where you can at least listen to it again, and then save money to upgrade in the near future.
Integra is Onkyo's "upscale" brand. This receiver for $150 is a great deal:
https://www.accessories4less.com/make-a-store/item/intdtr204/integra-dtr-20.4-5.2-ch-x-80-watts-networking-a/v-receiver/1.html
Get that and see how you like it with your Sony speakers, as KEW said. If you feel like you need more from your speakers, go for the highly recommended AA monitors. Those and the receiver together won't even have you $100 over budget.
All I'm saying is at least get to the point where you can start listening to tunes again, then save up for what you'd really like to have. A good tube setup will cost you, unless you find good deals locally.
