Despite agreeing to be bought by private equity firm Kohlberg, a better offer has now been made for 160-year-old piano maker

The final chords have not been played in the takeover of Steinway Musical Instruments.

The 160-year-old maker of pianos, saxophones and trumpets agreed in June to be bought by the private equity firm Kohlberg & Co but announced on Monday it had received a better offer.

But music lovers are in the dark about who has requested this unexpected encore – Steinway declined to identify the new suitor, describing it only as "an affiliate of an investment firm with more than $15bn under management".

This anonymous offer has topped Kohlberg's bid of $35 a share, which would value Steinway at $438m (£283m), raising the bar to $477m (£308m). Kohlberg, which said it wanted to accelerate Steinway's global expansion, now has the option to improve its offer.

The legendary company, founded in Manhattan in 1853 by German-born Henry Engelhard Steinway, is famed for still assembling its pianos by hand, long after rivals moved to automation. The musicality of Steinway pianos has won the company legions of fans, from Sergei Rachmaninov to modern concert pianists Vladimir Ashkenazy and Lang Lang. Romantic composer and pianist Rachmaninov described Steinway pianos as "perfect in every way", while pop singer Billy Joel used one to record his greatest hits such as Uptown Girl. Canadian wonder pianist Glenn Gould, who was obsessive about the perfect piano, played a Steinway Grand for most of his career, while John Lennon chose the piano to record Imagine.

Despite its reputation, Steinway has struggled in recent years as piano sales fell during the recession. Musical instrument sales have begun to recover: Steinway's net sales in the second quarter of 2013 were $92.4m compared with $85.7m for the previous year. But its brass section continues to struggle and bigger questions remain over its competitiveness.

Detractors of the Kohlberg deal fear Steinway's cherished craftmanship is at risk. Founder Jerome Kohlberg said in June the private equity firm would "accelerate global expansion, while ensuring the artisanal manufacturing processes that make the company's products unique are preserved, celebrated and treasured". Steinway, which has not been a family business since it was bought by the TV company CBS in 1972, announced it was selling itself last year, following a strategic review. The latest bid offer sent Steinway shares up 8% to $39.20 in early trading in New York.