MADRID — For two decades, Prisa’s solid Belle Epoque HQ overlooking Madrid’s Gran Via, the Spanish capital’s main shopping street, has been a sign of the media company’s stability.

That stability is now shaken.

On May 22, Prisa, which owns Spain’s most-read daily El Pais and largest radio net Cadena Ser, announced it was mulling selling “all or part” of satcaster Digital Plus, its biggest revenue driver and most-convincing claim to cutting-edge media modernity.

It has little choice. A e2 billion ($3.1 billion) bridging loan, syndicated by London-based HSBC Bank that allowed Prisa to hike its stake in Digital Plus owner Sogecable from 50% to 100%, is due June 20. So is at least $3.2 billion more in prior debt.

Prisa wants to keep Sogecable’s crown jewel, the fast-growing terrestrial web Cuatro, which turned profits in 2007 just two years after launch.

But it has already started to divest other assets. It’s just netted $487 million for real estate sales, including its Gran Via HQ and raised $77.3 million by selling its film and TV production unit Plural to Portugal’s Media Capital, which it co-owns.

Prisa is Spain’s biggest media company. But Spain’s still quite a small pond: Prisa’s 2007 cash flow was just $1.2 billion.

“Prisa hasn’t the money to pay all that debt,” says one analyst.

El Pais quoted CEO Juan Luis Cebrian saying a Digital Plus sale is one of many options it will have to consider.

The Spanish media are lining up suitors, including the usual suspects.

One is News Corp. Adding Digital Plus to its other Euro feevee assets — Blighty’s BSkyB, Sky Italia and Germany’s Premiere — News Corp. could play still harder ball on Hollywood product deals.

Others tipped to kick Digital Plus’ tires are Mexican telco magnate Carlos Slim, France Telecom and Spain’s Telefonica.

A telco could make a killing offering a triple play of pay TV, Internet access and telephony — particularly to Digital Plus’ 2.1 million subscribers who “represent the cream of Spain, with large disposable income for media and telecommunications,” according to Eduardo Garcia Matilla, prexy of research company Multimedia Corp.

“Offering clients a global telco package plus soccer could allow a telco to extract far more money from each sub than Digital Plus alone, which is offering just pay TV,” he added.

However, it’s unclear who will own Spanish soccer rights from 2009 — a pay TV driver in soccer-mad Spain just as it is all over Europe.

From the 2009-10 season Digital Plus could lose most Spanish soccer rights to film and TV company Mediapro.

The soccer rights are the subject of a labyrinthine legal dispute, which looks likely to be settled out of court.

If Mediapro wins, it will probably license the rights to Digital Plus at a much higher rate.

With or without soccer, suitors could be put off by the asking price for Digital Plus. Owing a lot, Prisa will ask a lot — maybe as much as $6 billion.

But there are few other such corporate deals to judge whether this is a good price or not.

Transactions that do exist hardly run in Prisa’s favor.

Early this year, News Corp. ponied up $844 per subscriber for 14.6% of Premiere.

Buying up all Sogecable in order to sell its Digital Plus, Prisa’s paying an estimated $2,320 per Digital Plus client.

CEO Cebrian has underscored Digital Plus’ high average revenue per sub: $881 in 2007 vs. Premiere’s $459. 

Yet sub growth potential — even from a low base — is limited (see chart) due to high pricing, piracy and a lack of VOD services.

Prisa will most likely try to buy time, extending debt deadlines or buying money, maybe via a bond-issue.

But one thing’s certain: the Digital Plus disposal will become a sell-a-thon.