Sustain is less a factor in evaluating a bass than other issues. First I would listen to the tone for richness (lots of harmonics sounding) color (subjective but a "desirable" tone), tuneability (smoothness of intervals), transition (from steel to copper and from bi to monochords) and power. There are few pianos out there that connote benefit from scale evaluation and modification to try to optimize the above factors within the limits of the design and piano size. The most important factors for me are transition, tone and balance with the mid and upper ranges. Hammers have a profound influence on bass tone as does scaling and mechanical integrity. Experience will help in distinguishing and separating different factors. Newton
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