Sell Hi-Fi and invest in stock market?

Hopefully I could get a large gain

To state the obvious, long-term the market is more likely to go up rather than stay down.

get a large gain and buy better equipment down the road

Hmmm, not so fast. You'll have to recoup the loss incurred selling yr equip used in a down market... And make enough to cover the added value of buying more expensive in a bull market...