Well, I don't believe that 531 tubing can 'get soft', but In theory a heat-treated frame like 753 could possibly 'get soft', and here's why:
My understanding is that 753 tubing was just thin 531 tubing that has been heat-treated, i.e. heated to high temperature an then quenched to rapidly freeze the grain structure in the random pattern obtained at high temperature, thereby stiffening the tubing:
http://en.wikipedia.org/
The opposite of Quenching metal is annealing, i.e. heating metal to a high temperature and then slowly cooling it back to normal temperature. That reorganizes the grain structure making the frame less-stiff but more resilient and increasing the fatigue limit.
http://en.wikipedia.org/
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Well, it turns out that the frame is annealing itself all the time, from the heat at room temperature. If you have ever looked at a piece of glass that is 100 years old, you can see ripples and lumps in the glass. This is not from a poor manufacturing process 100 years ago, in fact, 100 years ago the glass was perfectly smooth, but in the intervening 100 years the glass has "gone soft", i.e. it has integrated enough thermal vibrations into its grain structure to melt slightly and change shape, i.e. it has annealed, slightly.
The same thing is happening to Reynolds 753 all the time. The more heat the frame gets, the more likely the frame will 'anneal'. In fact, the degree of annealing is probably equal, almost, to the integral of all the heat the frame has absorbed since it was originally heat-treated.
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The conclusion is inescapable. First of all, don't buy any Reynolds 753 frames from Texas or Arizona! And secondly, when not in use, you should store your reynolds 753 bicycle in a refrigerated meat locker! Nothing less preserve the ride characteristics of your frame and keep it from going soft !!! :-) :-)
- Don :-) Gillies
San Diego, CA, USA